I see no issue in getting inspiration from other programs.

All projects do that, and I think that benefits the users.

Code has semantic highlighting via LSP, that is something KDevelop had before Code or Atom existed.

Same for the 'go to everywhere' that most likely was first in Sublime.

:) Perhaps even some features from Kate did inspire others, it is not that young.

because its not very easy to come up with new designs, at least for me. On the other hand if another tool provides a good and easy to understand UI, copying it makes sense. Kate still has a lot of its own things and ideas.

Which part is it copying?