I suppose now that it's been in Beta for a while, it's safe to start work on J4 versions. This is my 2nd most popular plugin, so I'll go ahead and get started. AdminExile is my #1 download, so I'll attack that first, followed by this. It shouldn't be much work.
Pls bear with me but I am not, by far, the best dev in the world. Nevertheless, I managed to update headtag for Joomla 4 and it works somehow.
Recently, I sent it to a private repo on Github and I'd be happy to invite you to take a look, hoping some of my work would save you some time.
Are you "stutteringp0et" on github ?
I did the update because I have been using headtag for a while and I needed it to test some stuff on J!4.
BTW, I began learning JS with mootools, and its study helped me a lot to understand it.