It is briefly mentioned in this post, but this work is based on a proposal by Paul Evans [1] (creator of libtickit and libvterm, among others).
Kovid deserves credit for addressing some of the issues in the original proposal, and for documenting it well. But Evans deserves credit as well for formulating the original idea.
Note also that this is not the only solution to this problem. Xterm has had its own solution for a long time called modifyOtherKeys [2] which has been supported by Vim and iTerm2 for quite some time (I believe foot supports it as well, perhaps others). Of course people have different opinions on which approach is “better”, though empirically it seems like the CSIu/“fixterms”/kitty approach has a lot more momentum at present.