There's no API to intercept web requests in a blocking fashion in MV3, so extensions can no longer offer the ability to do custom request blocking. The entire list is static.

> Are they also disabling manual install of extensions from outside the extension “store”?

This has already been the case for years unless you enable developer mode and load the extension as an unpacked extension (directory full of files rather than normal extension file) in which case chrome will complain every time you start it.