Wait, does that mean Manifest v3 is so neutered that it can't load a `<script>` tag into the page if an extension needed to?

If so, I feel like something that limited is hardly even a browser extension interface in the traditional sense.

Most browser extensions don’t need to insert script tags that point to arbitrary URLs on the internet. You can inject scripts that are bundled with the extension (you don’t even need to use an actual script tag). This is one part of manifest v3 that I think was actually a good change - ad blockers don’t do this so I don’t think Google had an ulterior motive for this particular limitation.

That is correct. You can not inject external scripts. You can fetch from a remote and inject through the content script though, but the content and service worker code is known at review time.

So you can still do everything you could before, but it’s not as hidden anymore