Rockband & Guitar Hero instruments still working on the next-gen Xbox series S or X?

No it won't work.

If you have older Xbox 360 instruments, there is a Rockband adapter where you can connect older Xbox 360 wireless instruments through that adapter.

But since the Guitar Hero Live controller has 3 pairs of "white/black" buttons instead of the five colored buttons, the Guitar Hero Live guitar is not compatible with Rockband 4. Please check

this compatibility matrix for which instruments are compatible with Rockband 4.

Hello CapitanYT,

Thank you for your post. I understand you have some questions on the compatibility of Rockband and Guitar Hero controllers with the Xbox Series X/S. Currently there is no information regarding the compatibility of these controllers, it is only known that
the Xbox Series X/S will support controllers that are usable with the Xbox One.

I hope this information helps.

Yes Rockband Guitars work on Guitar Hero games and vice versa. They are great for that price.

There is no Guitar Hero drums for the Series X or S. There is Rock Band drum kits for the Xbox One, which should work on the Series X|S consoles, but as SwordofWhedon said, they are now expensive, and can be very hard to find now.

Do you know if the guitar from guitar hero live on xbox one works on xbox series x?