We still have RHEL 6 machines in our data center. We aren't on extended support and the repos on our Satellite server aren't going to be updated.
Is there any reason to keep the repos on Satellite? I'm trying to think of one, and I can't.
You can remove those repos... whenever you're positively sure you will not need to:
a/ Reprovision a system with this older version of RHEL (because of a catastrophical failure that cannot be fixed with backups, etc).
b/ You will never need to install a new RPM package in those existing boxes.
Usually this only happens when the last RHEL6 system has been removed, but ymmv
Adding to Fran's comments, I'd keep them for as long as I have RHEL 6 systems running just to give me access to the repos in case of emergency (adding packages, DR, etc).
You can certainly stop syncing those repos though