In reality, this is something that people have been doing to systemd-resolved in desperation since the late 2010s (e.g. https://unix.stackexchange.com/q/421977/5132 for one of many examples), and systemd has not been changed in all of that time to undo this.

It is actually an unnecessary act of desperation, as systemd does have a way of setting things up so that /etc/resolv.conf is entirely manually configured. It's just (a) not completely obvious to deduce from the systemd doco, and (b) something that gets the blame for things that actually lie elsewhere in the system that also need disconnection, like /etc/nsswitch.conf giving nss-resolve priority over nss-dns.

That that situation remains the case today (five years after I wrote https://unix.stackexchange.com/a/612434/5132 for example, and almost a decade since the desperate making resolv.conf immutable idea took off) is a good indicator that the systemd developers aren't in reality in any hurry to force some hypothetical total ownership of /etc/resolv.conf by systemd.