Over the years have always been able to layer into Nik (free version) tools using shellout.py which still works with current stable 2.10.36.
I’m not a coder though and have been unsuccessful with my meager attempts to get Nik to work the same with dev Gimp currently 2.99.18.
Also can’t get G’Mic to work with 2.99.18 so far…
I put 2.10 plugins into AppData\Roaming\GIMP\2.99\plug-ins and see at least some of them flash by while Gimp 2.99 loads but can’t find anything in the interface…
No Python scripts for 2.10 can work on Ver. 2.99.x, because the specifications of Python-fu have changed significantly. And as far there are no appropriate instructions to rewrite existing Python script for Ver. 2.99.x. therefor no Python script authors can rewrite their script for Ver. 2.99.x.