don't worry about deprecated here, thats another problem
Sounds good.
... 3.20 is ok to target... If it was totally up to me I would say 3.24, but I know there are others using olde stable distros...
Looks like both Ubuntu 18.04 (EOL in 2023) and Debian stretch are using GTK 3.22. If someone really needs something from a newer (than 3.20) GTK release, it's easy enough to change the number.
You can turn deprecations on when building Geany to see how much there is, we had to turn them off because they were hiding real warnings.
Maybe I'll take a look at the deprecation warnings after #3019.