Hey,
I just noticed this as well, then found your post through my search engine, then was about to proceed in creating a new feature request on GitHub, but noticed I actually already did (Search input in community sidebar ยท Issue #1648 ยท LemmyNet/lemmy-ui), merely days before you posted this !
And maintainers have yet to respond to it, so I pinged them, hoping it will help move things along. ๐ค