For reference there is also a forum thread about this and maybe it's worth commenting there too since this issue has just been shut down as 'wont fix' and it's not possible to vote for a change of decision. I guess also that comments here now don't get seen by anyone but maybe it's possible to get someone's attention via the forums to reconsider this:

https://forum.unity.com/threads/cant-select-ui-elements-through-scene-without-enabled-gizmos.984792/

Please change it back, this "bugfix" really worsens things.

Only i work designing UI and this BUG is essencial for my daily work. If they can't fix it then i can't work in any of the new versions anymore

This was a frustrating regression to hit after updating from 2019.4 to 2020.3.

From a user's perspective I'm not it matters to us that "UGUI elements are not normal renderers", that's surely just a technical detail and doesn't seem like a clear explanation for why this bug is marked as "won't fix" - it almost sounds like it's been marked as "won't fix" just because it will be annoying to implement the fix :/

But what kind of argument is this? It was working in 2019 and that was clearly the better behaviour for many users. I don't think Unity should change behaviour that was relied upon for multiple years and then simply change it by calling it a "bugfix". Even more general, gizmos are things users can toggle on and off to add additional information and sometimes selectable things to the scene. It never disabled any sort of selecting GameObjects before, so why shouldn't I select UI GameObjects with Gizmos disabled?

This is also an issue for scripts that rely on OnDrawGizmos for drawing invisible mesh to create custom clickable selection areas.