The use of the word "maximum" here is not very clear; it's easy for somebody to mistakenly believe that this parameter limits the TTL of the higher caches to some maximum value. For example, if the second cache returns a hit with a TTL of 3600, then $maxLifetime = 60 would mean the first cache should use a TTL of 60 since that's the "maximum lifetime". But looking at the code, that's not what happens - this parameter actually works as a default TTL for items without an expiry.

I believe this possible confusion is why the variable was renamed in #26984 so let's update the documentation accordingly. Hopefully this will prevent others from making the same misunderstandings that I did :)