Kia ora Melissa,
I don't know about best practice, but this is how we do it.
When we archive a course we change it's visibility to No users.
This means it is no longer visible in the search but still appears on a learners ROL if they have completed it.
We also add "Z -archived" at the start of the course name.
This lets us find them easily in a report and shifts them down the results in the search for those that can see still them.
It also makes it more obvious to a learner that they have been archived otherwise the only notably difference is the course text displays greyed out. And if they click on the course link it will display whatever message you have set when someone tries to access a No user course
We don't use the archive function Craig mentioned as that sounds more like for a course that is still going to remain active.
There's a couple of older posts on archiving too. There could be more, but I just did a quick search:
Totara Learn Open Discussions - Archiving practice - how to retain access to all completion data?
Best practice forum (Archived) - How to 'retire' a course
Ngā mihi,
Larry