How good is it for gaming?
Perfectly fine, as long as you don't have any hardware, software, or drivers that depend on features introduced after whichever kernel version the LTSC release is frozen at (1809 for LTSC 2019, where 22H2 is the latest) Nobody documents this so you'll just have to try it for yourself.
Searching around casually, here's a thread about semiliterate people trying to run Cyberpunk 2077 on a comically old LTSB (predecessor of LTSC) Win10 frozen at version 1607 - that is, the kernel as it stood in August 2016.
Of course it didn't work and can't be made to work.
Combining information from a few different documentation pages you can get a rough idea of what DirectX version is supported where.
Win10 LTSC | Version | Build | DirectX Version |
---|
LTSC 2015 | 1507 | 10240 | 12 |
LTSC 2016 | 1607 | 14393 | 12 |
LTSC 2019 | 1809 | 17763 | 12.1 |
LTSC 2021 | 21H2 | 19044 | 12.2 |
If your favorite game comes out with an update that uses a version higher than what you're stuck on, you're out of luck and there's no workaround except upgrading Windows.
How much of a problem this is depends on what exactly you plan to play. If you're playing constantly-updated AAA games that demand the latest hardware and software you're probably going to have trouble sometime before LTSC's EOL.
That said, I've never had any issues with Steam itself (not that there's any guarantee they'll never break compatibility) and I don't play anything that comes remotely close to "cutting-edge", so I've never had any problems. Your mileage will vary.
EDIT: and if you're
building a new PC, be careful that your CPU is fully supported by whatever Windows kernel you choose. In particular, Intel chips with the "big.LITTLE" architecture (12th generation and onward) won't work at full efficiency on anything but Win10 22H2 or Win11. That would rule out any LTSC build that currently exists.