The inject_js thing isn’t really new; maybe it was just used from somewhere else before. Anyway, It’s not meant to be used from plugins.
To properly inject custom JS on specific pages, use a plugin and call self.inject_bundle(). If you only need it for customizations and want to include it on all pages, you could also use the js folder inside the CUSTOMIZATION_DIR and put it there.
Thanks, some of the JS I needed (addEventListener) needed to be placed at the end of the page, while the js folder places it into the header. I haven’t figured out how to write a plugin, so I will just continue with replacing the footer.html for now.