@nvertigo67 has already perfectly explained this, also in his recent response.
Just to add to this:
I did not actively disable stuff by means of "yeah, let's block feature 'xyz' " (unless I have put an option in the Settings leaving the user a choice), but I knowingly decided to include many hardening features from the GrapheneOS project into my ROM; which puts a significantly stricter limitation in place in regards of what the OS core components are allowed versus non-core components and 3rd party stuff ("user space"). The aim is to reduce the attack surface of especially yet unknown potential attacks.
The fact that Xposed, Gapps and other stuff cannot be flashed and/or do not work properly because of that is neither surprising, nor unwanted.