Mac Studio Display with 2015 iMac - yes it works....
I had two similar setups, one at home, one at work, both with 27 in iMac and a Thunderbolt Display. I recently upgraded my work system to a Mac Studio (very nice), but then only had one monitor. EMR work is much easier with two. So I brought my other Thunderbolt display to the office leaving my iMac at home all by itself. After reading that the Studio Display was not compatible with the 2015 on Apple's site and seeing postings that were contradicting that, I took the chance. Of course it works. Nice display. I am waiting to upgrade my home system to the Studio M4 Ultra and will pick up a second display for that. Apple makes such nice computers.