Some people provide contact details / a link to their professional homepage in their profile. People who don't do so may prefer not to be contacted by other people from the site. You can comment on other peoples' posts once you have gathered 50 points, cf. mathoverflow.net/help/privileges/comment .

@LSpice Comments can also be deleted independent of flags which are found to be helpful. -- In fact, most comments which are deleted are deleted because they are not needed any more, or because they don't provide much information, or because they are speculative or too chatty or for similar reasons. Inappropriate comments are fortunately comparatively rare.

From the timeline, I see that your comment got an "unfriendly or unkind" flag which was declined, and in the same time the comment was deleted.

@LSpice You are welcome! -- Done.

I deleted that comment in response to a 'no longer needed' flag, as it did not contain much concrete information (in contrast to Carlo Beenakker's response). Thus, to answer your question -- the comment was in no way inappropriate.

That is a good question. -- IMHO such edits should rather not be recommended, as they may leave the community (or the moderators) the choice between rather unsatisfactory options: have the question closed as off-topic with a highly upvoted answer, or reopen the question in a form which has not much to do with what the OP originally asked, or delete the originally off-topic question together with an answer which has substantial content. In the given case, with a bit of a bad feeling I have cast the 5th vote to reopen. Also, I have made the question CW (maybe the answer should be CW as well(?)).

I undeleted the question (the one found by Martin Sleziak). It was autodeleted, but it had no negative feedback.

@Negan Yes, it is always advised to report further violations if there are any. -- Every case will be carefully investigated. This however is no promise that the result of the investigation will always be what the person reporting the violation is aiming for. For example, sometimes rule violations may seem more severe on a first glance than they turn out to be in the end.

The question has been automatically deleted by the "Community" robot because it has negative score, no answers and is more than one month old -- cf. here.

Just a side remark -- deleting the question apparently caused a 100-points-bounty on the question from 2019 to be refunded.

The mentioned post is deleted now.

I said it was a case of doubt. -- That is, I am not convinced it is an answer. Rather -- in dubio pro reo -- the OP convinced me to give it the chance to be judged by the community, by votes. -- Not more.

That was a case of doubt. -- Following your flag, I converted the answer to a comment and moved the comments on it (since the comments contained information I felt shouldn't be deleted). -- Then, the OP of the answer explained in a flag why he thought it is an answer, and since the explanation sounded (vaguely) reasonable, I undeleted the answer and deleted the duplicated comments. -- Just that.

Rather than how difficult a question is, a criterion for a question to be on-topic on MathOverflow is whether it is interesting for mathematicians. On the one hand, very many questions graduate students may ask are interesting for mathematicians, and on-topic on MO -- but on the other, it is possible to ask arbitrarily difficult questions which are not interesting for mathematicians. -- "Difficult = interesting" is a common misunderstanding.