From my /etc/resolv.conf on Debian trixie, which isn’t using openresolv:

# Third party programs should typically not access this file directly, but only
# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a
# different way, replace this symlink by a static file or a different symlink.

I mean, if you want to just write a static resolv.conf, I don’t think that you normally need to have it flagged immutable. You just put the text file you want in place of the symlink.