The "closest" to documentation on how to write an Ubuntu MATE panel applet that I've found, so far, is the following topic here in the Ubuntu MATE Community, started by @anon94368460 on June 2019, where he explains how he created an applet to change the desktop settings for GTK 3 panel:
In that same topic, @vkareh (Victor Kareh - a MATE Developer) praised @anon94368460's post and added a few tips:
