I assume the reason it’s no longer on the marketplace is that there’s likely no point in installing from the marketplace any longer, now that the Quixel Bridge is part of the base engine. (Nor do I imagine the marketplace version would continue to be maintained as a separate thing, as a result.)
It might be that the binary build somehow mistakenly omitted it? Indeed, installing the binary build does not seem to have it for me (though installing the cached Marketplace copy of the plugin into the binary build works fine), while a source build does have the plugin already there.
This feels like an oversight/mistake to me.
In which case, until that gets corrected in the binary image of the engine, here’s a copy of the built version from the launcher’s local cache of Marketplace items on my machine. Just unzip it into the UE_5.0\Engine\Plugins directory (wherever you installed UE5); it should leave you with a Bridge subdirectory containing the plugin, and then I believe you’re good-to-go when you next start up the engine.
(Forum mods: I am assuming this is okay to throw out there to get folks unstuck, given that it should be part of the base installation – and demonstrably is part of the source build – and was free to start with; if I’m wrong on that and it is Forbidden for whatever reason, let me know and I’ll yoink the link.)