With Pi 4 units becoming more available, I was thinking about trading out my old Pi3+ for a Pi4. However, this is my 3D MVC Pi. So I've been lead to believe that I'll need to stick with the 9.2.8 build. What 4k/HDR limitations will I be looking at that far back? I looked around but couldn't find anything succinct.
There will be no HDR support with 9.2.8.
Personally I'd suggest you get a second sdcard with 9.2.8 for the rare occasions you play 3D videos.
And use a recent build the rest of the time.
In your case it's smart to have two RPi's: RPi3B+ with 9.2.8, and RPi4 with latest LE.
LE had massive improvements for 4K/HDR during the last years, but none for 3D.