Auto-Dig itself doesn't declare any Space Age feature flag and doesn't depend on space-age — you can check its info.json, it's just base >= 2.0 and the-cave >= 2.7.0.
I tested it headless with the Space Age data removed entirely, and the-cave + Auto-Dig both load and generate a map fine, so nothing here needs the expansion's content.
That said, the-cave sets space_travel_required in its own info.json, and I can't rule out that this gates on owning the expansion on the client side — I have no way to test that here.
Could you paste the exact error you're getting, and whether it appears in the in-game mod manager or when connecting to a server? That would tell us straight away which of the two it is. If it turns out to be The Cave's flag, that's upstream and I'll raise it with its author.
