Buster has been out for several months so PiHole on Docker should be rebased to use Buster instead. The version of PHP on Stretch (7.0) is no longer supported.
I commented on this here : https://github.com/pi-hole/docker-base-images/pull/14#issuecomment-573166183
The entire thread has a few different takes.
tl;dr we’ll get around to it in a while with proper testing/vetting. Security support for the old OS being dropped will be the impetus to force us to upgrade, we’re in no hurry since we have a big of security patching support time still.
I already have PiHole running on a spare VM running Buster and it works just fine. However, my main PiHole instance is a container and my primary reason for wanting this to happen sooner vs. later.
While understanding that security is ongoing for Stretch for a while, PHP 7.3 is ~30% faster than PHP 7.0. https://www.phoronix.com/scan.php?page=news_item&px=PHP-7.3-Performance-Benchmarks
From the GitHub thread that you posted, it looks like DL6ER has confirmed that there should be no technical limitation to using Buster.
To be clear, it'd be one thing if this was just the container to build the images. However, from trawling through the code, this looks like what is also included when running in a container for end-user use.
Lastly, I'll leave some usage numbers across PHP versions here.
Buster variant is available now, the more people testing and commenting that they have no issues the more confident I'll be to make it the default