Do we need a new mod for the no space age variant?

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.

First of all thank you for your time and effort! Now behroozzx has made two variants of the cave (with - without space age). Yours is limiting it to the space age variant only. Maybe you should put both but as optional prerequisites.....