As of 2026, I doubt that the core PuTTY team will ever finish one.
We've tried more than once. Around 2005 we attempted one in the form of a native Cocoa application, but it turned out to be very slow to redraw its window for some reason we never got to the bottom of.
In 2015, after porting the GTK front end to work with GTK 3, we began another attempt, based on making small changes to the GTK code and building it against the OS X Quartz version of GTK 3. This didn't have the speed issue, and came much closer to being finished. But unfortunately the Quartz GTK port seems to have bit-rotted: the last time I tried to build the Mac port, I couldn't even figure out how to install a GTK it would build with.
If anyone were interested in picking this up and finishing it, the TODO list in unix/main-gtk-application.c in the source code is a good place to start, and I'd be happy to provide advice and discussion.