They won't come out and say it or confirm it, but I'm pretty sure that's what they did, as they just made it backward compatible only. On the XBox One X, PGA TOUR 2K21 looks and flows at the same 30 FPS @ 2160p, as TGC 2019. The PS4 Pro version of TGC 2019 was natively rendered at 1600p @ 30 FPS. They were quick to add that it got upscaled to 4K. Two weeks later they wiped the forum and lost that post (I have it in my archives and shared it to at least three other forums), and haven't said a word about resolutions, frame rates, or anything relative to graphics quality.
I've got a GTX 1070 and I can play the game at 50 to 62 FPS on most courses in 4K. It makes a HUGE difference in the way this game feels and looks. I think it's essential for proper enjoyment and management of the game's tempo component.
It's currently strapped to 30 FPS on both next-gen consoles. You're asking all the right questions and I can tell you this; as the owner of both the PC and XBox One X versions of the game, it's not worth buying it for a PS5 or an XBox Series X, since they're at least currently shackled to 30 FPS, probably because HB Studios has the vsync locked at 30 Hz. I'm hoping they're going to unlock the vsync so PS5 and Series X owners can play the game at 60 FPS and get the benefits of Variable Refresh Rates of their consoles and their VRR capable displays. It would be such an easy software switch to add.
I've quit playing the game on the XBox One X, even though that's where most of my friends play the game. It's just too frustrating trying to manage the critical tempo component at 30 FPS... not only that, but it sort of feels like playing underwater compared to the unrestrained feel of 60 FPS!