There's not much up-to-date information about creating icon themes for Android out there, so this document will collect a few useful tips and links.
First of all, I'm using Lawnchair as my main launcher, so that's obviously the one I'm testing with. Lawnchair is open source which has been incredible helpful!
For example there's a list of icon pack intent names
(source for v10, source for v11)
for which <intent-filter>s should exist, and from the appfilter.xml parsing
code (source for v10, source for v11)
we can see how that file should look like and how dynamic icons like clock and
calendar can be created.
There's some existing icon theme code as well, see
iamareebjamal/scratch_icon_pack_source. Notably it
includes an example appfilter.xml, drawable.xml and AndroidManifest.xml.