Im pretty sure some of my really old WP's were around pre 80 but dont think pre 78. Marzocchis and Thermal flows are the obvious ones plus a few oddball euro brands but other than there wasnt much that i know off
. The ohlins piggyback were on the 79 works suzukis from photos ive seen but i dont think they were available to the public till 80 like moto says. Fox Factory piggybacks appeared in 1979/80. I think everyone knows modern piggybacks shouldnt be allowed in pre 78 but they are. The rules wont change though so not much point getting too worked up about it now. Ideally the rules couldn't have be written to allow the old Marzocchis but not modern piggyback Ohlins etc. I say just use what ever shocks you want to.
I don’t know of anyone making new shocks with remote reservoirs which is why you see so many piggyback shocks these days
Companies can make them, usually by special order though. Ohlins could make them if they really wanted to. They have the parts. People just seem to favour the piggybacks these days though and demand for remotes has dropped off. There is only 2 key parts that are no longer made that is stopping them making them at the moment, thats non adjustable ones. They can be easily made if you have a lathe though