Mamba wrote:

....
Does old propane go "bad"? When I removed some of the old piping, there
seemed to be a nasty smelling gel layer inside the pipes. Is this a
congealed version of propane? Do lines need to be reamed out before
reusing?

Propane does not go "bad", but (IIRC) the mercaptan odorant can.
Your local propane company will check that as part of their
service initiation inspection.