What decides for an anime announcement to get a sticky
Depends on the mod.
I remember Chihayafuru S2 got a sticky.
It has to be something that was meme-worthy for a very long time a lot AND unexpected so that lots and lots of people on /a/ would talk about it.
One Punch Man getting a second season was never unexpected at all. Ever.
Berserk got a sticky for when the party finally left the boat, because the party was on the boat for nearly a decade-long meme.
Luluco getting a sticky for its final episode when it was announced that Little Witch Academia would get a TV series was because LWA is very popular among many on /a/ and it was unexpected.
One Punch Man season 1 got a sticky because it getting animated was unexpected, like LWA.
Full Metal Panic getting a sticky was because there was years of no announcement of any continuation to it.
If Code Geass were to get a confirmed TV sequel, that would be for example sticky-worthy.
Maho got a sticky