As of Big Sur this is no longer really feasible.
The root filesystem on macOS has been protected strongly forever, with each of the last few OS revisions increasing protections (with SIP, etc)
macOS mounts the System area read-only and as of Big Sur makes sure to only trust it if it has not been changed.
If you violate that trust, certain parts of the system won't work again (specifically: you will not be able to re-enable FileVault afterwards)
If deleting the *.heic files is that important to you though, follow this guide Can I mount the root (system) filesystem as writable in Big Sur? (including it's warnings!)
Then from Recovery you may
rm -f *.heic /Volumes/<your hard drive name>/System/Library/Desktop Pictures
I really wish I had that space back, but I need FileVault more so I gave up.