If they actually had checked the github they'd have found that there has been activity recently, including a working release. https://github.com/darkfiggy/dmaxplugin/releases
I did make a fixed (as in: it compiled and PKHeX loaded it correctly) version of the plugin back in March. Still seems to work on the current version. https://github.com/BtEtta/dmaxplugin
I should have "fixed" the Dynamax Adventure Plugin. At least in as much as what worked in the first place should still work.
https://github.com/BtEtta/dmaxplugin/releases