[selfcontained]

I am expecting delivery of a Khadas VIM3 Pro in mid-January. I want to install CoreELEC 9.2.8 instead of the latest CoreELEC 19.3 because I want to use Kodi 18.9 (because there is a plugin I like that hasn't been re-written in Python 3). I plan to get a Samsung HDR10+ capable display and playback HDR10+ encoded videos.

Is there anything Kodi 18.9 Leia cannot do that Kodi 19.3 Matrix can (as far as HDR, etc. playback)?

tl;dr - what visual playback features does Matrix have that Leia does not?

[Klojum]

what visual playback features does Matrix have that Leia does not?

In short, Kodi 18 has very minimal, if not any, capabilities in displaying HDR properly. Kodi 19 got more development in that area.

I want to install CoreELEC 9.2.8 instead of the latest CoreELEC 19.3 because I want to use Kodi 18.9 (because there is a plugin I like that hasn't been re-written in Python 3)

Have you already contacted the CoreELEC forum with these questions, since .. this is not the CoreELEC forum?
Personally I think CoreELEC is not going to bother backporting full HDR(+) support to their Kodi 18 release.
Perhaps reviving that one Python-2 plugin is a better bet.

[selfcontained]

I have asked the same thing on the CoreELEC forum - no answer yet. I expect the answer will be the same as yours though and I'll just use Matrix like every other sane user.