I am also having this issue, as are a bunch of people online... https://www.reddit.com/r/ios/comments/1fmql4y/imessage_stickers_have_disappeared_on_ios_18_16/
Hopefully they do figure out a way to push a fix, it is a fun feature, and I would be fine reseting the database of stickers, but it is a bummer to not be able to use them at all.
aduss
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