Archiving legacy metrics repositories

Today I run into an issue where I was going crazy understanding why a submodule for metrics projects was not picking some changes made a few months ago and then I realized it was still set to git.tpo which is not being updated. I am hence thinking to archive all metrics projects on git.tpo and just use gitlab. One thing I am also thinking is that there is valuable knowledge in git.tpo and so I was wondering: what is the procedure to archive stuff on the old git? Just stop updating the repositories? In the past the idea was to just move stuff to the attic, but that made things confusing, in my opinion. Is there a setting to make these repositories read-only?
I could just set the setting in gitolite that no one has permission to write to those repositories. Is that ok?