Water Hammer

Better to have them in place and not need them then to take them out and find out later you needed them.

I like them in, new code says you don't need them unless on a mechanical valve ( dishwasher ice maker etc,) i still use the old bullets on each bath group never hurts. keep them do not replace with the new mechanical type. looks like you may have had a three or two handle set up which sat lower than the single you have. the hammers should be above the servicing valve and should drain empty when turning the water off in the system. also do something with that galvanize. piping.

Thanks, I am going to remove as much of the galvanized as possible and replace with pex. Space is very limited, so I cannot replace all of it, but the shower, toilet, and sink supply lines will be replaced

Non-mechanical water hammer arrestors eventually fill with water and lose their effectiveness. This is because the air cushion is gradually absorbed by the water. If you drain the plumbing system completely (turn the hot water tank off first, although you don't have to drain that tank completely too) the arrestors will empty out also. Twice a year should be sufficient.

Leave the water hammer arrestors in place. You can make a new arrestor out of either PVC or metal if you need to replace the pipe section containing one. It's simply a seemingly useless upward pointing stub pipe.