Let me first say that I'll try to answer your question generally, not to concentrate on the specific user you're looking for. So I understand the question as: "Is it possible to find user account in situations similar this?" (Also, I will try to answer this from a viewpoint of regular users. Moderators might have some more tools at their disposal. However, if you think that you were subjected to serial downvoting and want mods to investigate, flagging might be a reasonable way to ask them to alert them to the situation. But in the situation described above this is a moot point, since the votes have already been reversed. And this is only marginally related to your question - since in such situations mods will possibly revert the votes if you were serially downvoted, but they will not tell you identity of the user - which seems to be what you're asking about.)
I'll also add that trying to find out who cast downvotes on my post is, in my opinion, not a constructive use of my time. But maybe some of the stuff mentioned below can be useful in some situations even when looking for useful information.
The third thing I should mention is that one of the key features of Stack Exchange software is that voting is anonymous. (And this is unlikely to change.) Although in some situations you might guess who downvoted/upvoted your post, you cannot know for sure. (To mention specific example to show what I mean, when I look at the OP's profile here on meta, I see how many upvotes/downvotes he cast on meta until now. And if later today this answer is at $-1$ and I see that counter showing number of downvotes on his profile page has increased, then it's quite natural to suspect that the downvoter might have been him, but it is very far from a proof.)
After the disclaimer above, let us have a look at your question, which can be interpreted in several ways.
If my reputation tab shows a change of reputation saying user was removed, is it possible to find out which user was it? No, if user's account is deleted and in situations where this leads to removal of votes, you cannot find out which specific account was removed. This is discussed in more detail here: Is there a way to know which user was removed?
Is it possible to find a list of all users that were deleted recently/during this week/during some specific time period? As far as I know, this does not offer you possibility to do this. But you could the data from data explorer and the fact that they are only updated once a week (Monday morning around 3:00 UTC). This means that some stuff which was deleted can still be in the database until the next update. (I think it might be reasonable to add additional restrictions so that you do not have too many results - there is a limit on number of results of SEDE query; SEDE returns only first 50000 results if the query returns more results than that. In this case, you say that users' reputation was above 1000, so you could use this query. Or you can try to restrict your attention on users above certain number of votes.) You can download the results - and then simply sit and wait, download results of the same queries during the next week - after the update of the data in SEDE - and compare them. You will get list of the users that were deleted.
Which are the posts on which I lost/gained reputation? You did not ask specifically about this, but if somebody sees that some of the upvotes/downvotes on were removed, they might also be curious which are the posts in question. (The reputation tab simply says "User was removed" without showing that exact posts where the vote tally changes.) Here I cannot offer any better suggestion than to occasionally download the breakdown of your reputation by post which is available here: https://mathoverflow.net/reputation By comparing two versions from various dates you might be able to find out the post which gained some downvotes/upvotes in that period.
I had exchange in comments with some user, now the comments are deleted. Can I find username/user account? We will have a closer look at this problem below.
How to find a user I interacted with - if the comments were deleted?
This is actually the part question which I can consider useful. It is possible that you had interaction with some user and you want to find that user - you have something to add. But the comments were, for some reason, deleted.
It definitely helps if you remember under which post you had the comment exchange.
Some things that can be tried are:
- Check your global inbox. Sometimes, but not always, deleted comments stay there.
- Long exchanges in comments can be moved to chat (automatically or by moderators). If the comment exchange you're looking for was rather long, it might be worth looking in chat. You can have look at rooms imported from main site. (Those are the rooms with the title "Discussion between userA and userB" and description started "Imported from a comment discussion on ...") Or you can look at your chat profile and check the recent messages posted by you. (If a conversation you were part of was moved to chat, the comments you posted in that conversation will also appear as chat messages in that room.)
- If you remember on which question you had the discussion, you can check whether the comments can be found using Wayback Machine or whether they are still in Google Cache.
- And you can also try to use data explorer - assuming that the deleted comments were already there before the last data dump, but the data in data explorer have not been updated since the deletion. (You can look for comments on specific post or for all comments under your post, etc.)
Of course, in neither of the above solutions you can be sure that you actually find the deleted comments. But if you are really looking for something, they might be worth trying. I will add link to this discussion on another meta: Are there some possibilities for users below 10k to see deleted posts of other users? In an answer to this question you can find a SEDE query which shows comments on a specific posts: Why were the constructive comments deleted along with one poor comment? If you would like to make a query looking for all comments on your posts, you can try to modify some of the queries here: Is it possible to search for posts/questions of an user $X$ commented on/answered by the user $Y$?
I should add that I can see that some users might see investigating stuff like this problematic. If a user decided to remove their account, it's basically their personal business. (And if it was done by moderators, then the reasons should be kept private, too.)
On the other hand, I also understand the natural curiosity - many users saw a recent reputation change citing "user was removed" on their profile page, so they might be curious what actually happened. (Although it seems that quite often - unlike in the case of the OP - the reputation change is negative, which suggests that in those cases upvotes were removed after the deletion of a user's account.)
In my answer I described some tools how to get an information about some deleted stuff - but all of this information comes from publicly available sources. So I hope that the answer I posted is acceptable even for users who have dim view on looking at accounts' deletions by regular users.