I havn't had time to get back to #2860 so thanks for this.
When you say "removed properties" did Glade remove them or you remove them?
I am not sure what is the earliest GTK Geany targets, but GTK3.20 was early 2016, so nearly six years, which IMHO is reasonable enough since most major distros only provide 5 year LTS AFAIK. If someone needs Geany to support an older version of GTK they can run a branch. The last thing we want is to return to the situation where contributors have to use old versions of Glade that their distros don't provide (although it did reduce the number of glade file changes :)