Most likely veterans became bored with Awfully sophisticated proof for simple facts but for newcomers it is such a delight!
My actual question is: what happens with closed questions? Do they become harder to encounter with time?
Some information about closed/on-topic questions can be found in the meta.SE closed/on-hold faq.
There are two important points about closed/on-hold questions.
Another side effect of being closed is that such questions may become eligible for deletion, either through delete votes by users with at least 10K reputation, or in quite restrictive cases automatically by the system. (Moderators can vote to delete any post at any time, and post owners can "often" unilaterally delete their posts. There are also very restrictive cases whereby non-closed questions can be deleted. More information about all this can be found in the meta.se deletion faq.) There are some who view being closed as the first step towards a question's eventual (inevitable?) deletion.
Closed questions will still appear in search results and other question lists, so you can still come across them in the wild. (The
closed:1 search parameter explicitly requests only questions which are closed/on-hold.) When any post within a closed question is edited, this will bump the question to the front page as usual. But as no new answers will be posted, it is reasonable to expect that they won't appear on the front page as often.
For the particular question linked to, I have nowhere near enough reputation here on MO to vote to close questions, and clearly was not involved in the closing of that question, but I suspect that users felt that the question had run its course and was unlikely to elicit any new, good answers. If this is indeed the case, I almost wonder if that question would be better off being "locked for historical reasons" than closed.
This question is not in danger of being deleted any time soon. It does not meet any criteria for automatic deletion, and it would require 10 delete votes to be deleted.
The technicalities of closure are described in Arthur Fischer's answer. Let me comment a bit on the practice that caused such closures.
Specifically the closure of this questions seems to have been induced by its asker (Mariano), via the following comment on the old meta board:
Sadly, the «Awfully sophisticated proofs» question should probably also be closed...
I really wish people were more selective in what they add to list questions. I enjoyed that thread a lot.
I have just cast the final vote to close on my own question about proofs without words.
Lately, my efforts to keep it... hm... as great as it started have not been enough.
Generally, it is or at least was common to close "big list" questions after some time when they had gotten many answer. A principal reason for this is that empirically there is often a decline of the quality of new answers and especially a proliferation of duplicate answers starts, as some new answerers seem not to bother to read the existing answers before adding their own.
Furthermore, some are of the opinion that such questions are off-topic in the first place and to control the number of open questions of this form was some kind of compromise solution.