I still find it quite sad that the pinnacle of GUI programming tools was reached over 20 years ago, when Delphi 6 was released, and we've been going more or less downhill ever since.

I see it differently: UI needs have gotten more complex. Like how we used to have web sites that said "best viewed in 800x600 in Netscape Navigator" we rarely design one size fits all UIs any more like we did in Delphi.

There's nothing in "modern" UI that cannot be done with Lazarus GUI Builder: Dynamic layouts? check, anchors? check, flexbox? check, custom components? check nested hierarchies of elements? HIDPI? check Vector Graphics? check Multiresolution? check and so on. All of them compiled to native (and looking native) on every platform under the sun. As said before, GUI design peaked in the 90's

Does that extend to internationalisation or accessibility?

(Real questions, Lazarus was before my time)

Honestly, all the momentum went to web UIs...desktop UI R&D then just died more or less.

Technically the pinnacle was around 2010.

Guess what happened around that time?

Firstly, the web won.

Secondly, mobile cemented the demise of the desktop.

Especially since mobile is horribly anti-crossplatform GUI toolkit, especially against open source toolkits.