I would say if someone installs it on something that can be called an 'Apple branded computer' in 50 years time when Macs are long forgotten, it's probably legal. Current versions for example allow running the software on 'Apple branded computers', while slightly older versions allowed running on 'Apple labeled computers' (probably changed because some joker put an Apple sticker on a Dell computer and claimed it was 'Apple labeled'). You'd have to dig out an old license agreement and read it very carefully to see what is actually allowed. You certainly cannot buy MacOS 1.0 for money.
I think that was true with versions up to 7.5.3, possibly 7.5.5 I've never seen any version before 8.0 for sale. Back in the day, many MacOS versions were not sold, but available for free - if you installed it on hardware that shipped with MacOS X (that was in the days of clones that shipped legally with some MacOS version).
