Hubs that accept a Shimano 11 speed cassette will also accept Shimano 7, 8, 9, or 10 speed cassettes and shifting should work fine with the proper spacer in place on the hub. The 8 speed should, if I recall correctly, need a 1.85 mm spacer. And otherwise the trainer has nothing to do with the rear shifting. As long as the 8 speed derailleur and the 8 speed cassette are lined up properly, the shifting should be just as good on the trainer as outside. An lining them up is just a matter of the spacer.
With that said, it is nice to have a dedicated trainer bike. It’s easier to maintain the habit of riding if you have minimal extra work to do to get on the Zwift roads. I’d just suggest that when you’re looking for a cheap bike to keep on the trainer, don’t rule out used road or hybrid bikes going back at least to the 8 speed era.
Our trainer bike is now a compact double 11 speed drivetrain mounted on a cheap frame. With the dropped bars and brifters it is more like riding outside. But the old triple 7 and 8 speed MTB and hybrid we’ve had on the KICKR as dedicated trainer bikes over the last five years provided better gear choices than this compact double does.