Is the steam family sharing user cap a life time limitation?

You may authorize Family Library Sharing on up to 10 devices at a given time, and for up to 5 accounts that may then use your game library on any of your authorized computers.

From this it is unclear to me whether I can add and remove up to five friends continuously, or am only allowed to add five friends and that's it.

I would prefer some kind of source with an answer.

Thanks in advance.

You are allowed to add them continously. However there is a constraint, namely that you are only allowed to add 5 friends roughly every 90 days (So when you added your 5th friend you have to wait 90 days until you are able to again. Further you can't trick the system by deleting friends before the term).

You get a message when you are in this situation which I don't want to reproduce at the moment because of the 90 days - That goes without saying :-)

Steam can't say when you have shared your games with 5 friends it's over before warning, but also it is self-evident that there has to be this constraint with the 90 days since piracy would take place in form of shared steam libraries in combination with virtual LANs (which is existing already what I strongly assume). The same issue you have on the market e.g. with time penalties on item deals.

There are several post in the steamcommunity sites, but you might've a look into this one.