It’ll just skip over and treat it like you’ve done both I believe. That or it stops the mountains biome raids. Not really sure. Bonemass is definitely the most optional boss of the bunch though. You can find plenty of silver without him and most other things you can find with the wishbone are crap/optional. Definitely won’t break anything.

Technically speaking, Moder’s drop is the only mandatory one to unlock things. Getting Iron without defeating The Elder is a chore and a half as it’s a low chance drop from Oozers and can be mined in small quantities underground in swamps, which are hard to find without the Bonemass drop. If you cheese enough copper using a Troll, you can get the copper needed to make a pick without Eikthyr. However, there’s no alternative to killing Moder for the special bench to get the final tier(for now) stuff.