The eldar was a very special character, as the rules for playing orks, that came from a time when the "necromunda magazines" were more alike to a fanzine.I don’t know that Xenos weapons have ever been particularly hard to get on Necromunda. There was always an active Eldar and Xenos community in one of the smaller parts of the spire and the Spyrer tech always had Xenos implications to it.
The bigger issue with N18 is that rare weapons in general are just much easier to get than they used to be. If all this was in some sort of rare trade chart that you only rolled on once or twice after every game with a diminishing chance of this sort of tech it would be ok.
But that seems like a heretical idea to the current zeitgeist of wanting all the toys.
And the spyrers suits is true that GW retrocon to have TAU technology, but the spyrers themselves and their equipment are a matter of dark legends for the typical underhiver (their owner will be the equivalent of a crazy collector noble from other settings)
Having rules for xeno technology is not a problem, but making it available for isolated imperial citizens like underhive gangs is. For a campaign set in a space station hub, like precipice (from blackstone fortress game) where a lot of species meet will be a great addon.
It they want to expand Necromunda itself into something more diverse I think the perfect option is the Navis Mortis, a giant spaceship graveyard where crash smugglers ship shot down and alike.
There you can introduce a lot of weird equipment as treasures to find, xenos contamination (even orks) and even alien strike teams from a crashed ship or a rescue mission.