4.19 Flex from Nvidia repo I assume? I made them PR for but they haven’t accepted it (/pull/523)
I have fixed branch due to the open PR : https://github//UnrealEngine/tree/PR-FleX-4.19.2, additionally my contains FleX and fixes as well.
For 4.20 I have a separate FleX branch : https://github//UnrealEngine/tree/4.20-FleX and https://github//UnrealEngine/tree/4.20-GameWorks if you want other techs too.