Who does the EOL notices, and why are they posted on meta and not the normal site? It seems they are all done by random people with high rep.
You've answered your own question: anyone does it.
It's just an announcement to other members of the site who are active and read Meta: a reminder that we should start closing questions related to the now-EOL version.
Posts like that don't belong on the normal site, as they are about Ask Ubuntu, not Ubuntu. Only Ask Ubuntu doesn't support EOL releases. U&L doesn't care. Ubuntu news doesn't belong on the main site; it belongs in chat or Meta. The main site is for questions and issues involving Ubuntu, not Ubuntu news (not a black and white situation, but it applies here).