That doesn't explain why other races don't discover it and use it.
True, but you could say the same about a lot of the abandoned technologies that litter the Trek landscape. If the guys from DS9: "Battle Lines" could invent nanites that could cure every mortal wound and make people immortal, why has nobody else invented them? If Rao Vantika from "The Passenger" had a technology to preserve the brain after death and transfer it to a new body, why doesn't any race more advanced than his have the same technology? If Platonians, humans, and Vulcans could develop telekinesis by injecting themselves with kironide, why isn't there a huge galactic market in kironide mining and distribution to give everyone telekinesis? Why are the Sikarians from VGR: "Prime Factors" the only ones who ever invented long-range space-folding teleportation?
Yes, it's a plot hole, but it's a category of plot hole that has already existed in
Star Trek for over half a century, so it's not like there's anything new or unprecedented about it. Heck, it's not even unique to
Star Trek. Universe-changing technologies that get forgotten after one storyline are a standard trope of episodic science fiction series.
Also, wasn't it specifically the way Terran Empire was using it that was damaging it, not the Federation's?
Hardly matters. If it's even
possible to destroy the multiverse by damaging the mycelial network, then that's reason enough to avoid interfering with it in any way, for fear of doing unintended damage or causing an accident or something. The stakes are so high that it's just not worth the risk.
And it would seem like a good candidate for Voyager to dust off for one quick hop if the specs were in the database. Unless the lame spinning saucer is mandatory and not just silly fx.
How could they do that? It takes more than "specs." It takes a supply of
Prototaxites stellaviatori fungi, plus either a captive "tardigrade" life form to torture or DNA therefrom to conduct illegal genetic engineering with.
Voyager had access to neither the fungi nor the tardigrades, and we know from "Equinox" that Janeway drew the line at torturing sentient beings to get home faster.
