As noted by Ulli in https://github.com/jenkinsci/jenkins/pull/5916#pullrequestreview-829675814

One thing that I really don't like is the position of the search bar. It is far too wide in the upper right corner (where it looks unrelated) and a lot of users will not find them when opening the new page the first time. It already is not ideal in the current UI (where it is above the tabs rather than right above the table), but now I think the usability is getting worse.

As the search bar is the major UI element that admins need to use for the Available tab, given it only lists a tiny subset of plugins (50 out of ~2000), this is a choice so poor IMO it's a regression.

The search bar should be big and centered on this tab, almost like search engine home pages, not far off to the side.