The most fascinating aspect is that the old members do play along and appear as if they don't know what's going on, as we have today, many threads have been opened for questions, and some other members who offer signature service management have made jaw tantalizing rates threads to fool participants.
Theymos always appear serious about it with the support of some extremely reputable forum members, which makes the joke interesting. Most of the time, the jokes are always about something very difficult and the majority of forum users will not be happy if it is implemented.
Kudos to OP for taking the time to compile all of these Theymos April Fool's Day events from 2015. However, this Captcha is challenging because the more difficult the next one is to solve, the easier the first one was.