It would be great it people would pay Rui to make mold versions for Windows and Mac, which ideally would be required before making it a part of the official Rust tool chain.
Also, mold was designed as an alternative to gold / lld, therefore it would require to be open-source and free on their main platform: linux.
It's AGPL on Linux now, and they sell commercial licenses for companies that won't touch that license, and they were contemplating earlier making mold only available under a non-free source available license like BSL, so there's no "requirement" as such that it be free and open source, even on Linux.
Do you have any data on this ? Maybe that's industry dependent, but I hardly know any Windows (not even talking about macOS, that's almost nil) developers outside of video games and web dev. 100% of Rust devs I know use Linux, to keep on the subject.