i thought it [animated avatars no longer allowed] was because the board got hacked via an animated avatar exploit back in the day (like ~2011-15?) but cant find anything with a quick search.
I don't know what the facts are. I wasn't even registered here when all that went on but I've stumbled on some monikers with animated avatars here. Some of the users who still have them are enter no longer using those accounts for whatsoever reasons or the owners are deceased. By the way, between your thought that it was stopped because it enabled "the board" (whatsoever that means) to be hacked and that loading it on the site takes enough of forum time, I think I would go with the second explanation.
board = forum.. a hold term over from bulletin board systems from the dial up modem days
anyway, found it.. check quotes from 02 oct 2013 by @theymos here
Also, the attacker was able to upload a PHP script and some other files to the avatars directory.
https://cryptolife.net/bitcointalk-hacked/and at reddit
https://www.reddit.com/r/Bitcoin/comments/1nmdq4/bitcointalk_hacked/and here on the forum
https://bitcointalk.org/index.php?topic=306878.msg3290091#msg3290091unsure if that was the final determination however. its a bit off topic though but still interesting