Do you know how the GoG version handles achievements? I'm very used to the Steam achievement system, but obviously that's not an option when you're not using Steam.
Hollow Knight had an internal achievement tracker so it will likely be the same for Silksong. You could even unlock achievements on other platforms by porting your save file over.
Is there an aarch64 linux build by any chance? (I've been playing the original via muvm/fex, which mostly works but crashes in menus frequently)
The system requirements list an x86-64 processor, so probably not.
I don't know any commercial game that offers that. People playing games on arm64 Linux is a niche within a niche.
It is a niche, but given that they already have aarch64 builds and linux builds, an aarch64 linux build ought not to be that big of a deal.
I guess, but linux/arm64 specific bugs can and do happen. For my open source projects I offer builds for arm64 and others because, well, why not? I don't test it and I can't really test it (outside of qemu, I guess) and if it doesn't work then that's your problem to at least debug and probably fix too. But that attitude doesn't really work for a commercial game.