I am afraid no plugin was made, it would be a very complex plugin if it was to retain the old functionality.
We used to use the library: Gifsicle: Command-Line Animated GIFs to resize the animated avatars, this dependency was removed.
To implement in a plugin there are a few options
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You could pull in the dependency somehow, very complex especially from plugin, then patch the code to support this in a plugin (extremely hard)
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You could give up on resizing animated avatars and add a patch to a plugin (medium)
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You could figure out some other way of resizing animated avatars and put that in a plugin (extremely hard)
No experimentation here is queued from the Discourse team. We have completely abandoned the idea animated avatars for now.