I havn't had time to get back to #2860 so thanks for this.
Glad to do this. Figured there's just too much for you to do.
When you say "removed properties" did Glade remove them or you remove them?
I removed them and resaved. The files saved by Glade 3.38.2 appear to be stable.
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 :)
The problem is ... Targetting lower than 3.20 produces those "introduced in" messages. Targetting higher than 3.0 produces deprecated warnings. I didn't know the correct approach, so just picked one and wait for feedback.
I just found some visual differences, and it does look better with the properties set. I'll put them back and target gtk+3.20.