Windows Server 2019 FFL/DFL
Q: Where is Windows Server 2019 FFL/DFL?
A: There isn't one. Since there was no updates or changes to the Forest or Domain functional levels of AD, there is not a 2019 level, but only a 2016 level. This is the information about Windows FFL/DFL's: https://docs.microsoft.com/en-us/windows-server/identity/ad-ds/active-directory-functional-levels
This image also shows the various paths and options for upgrading and downgrading FFL/DFL's as needed.
The biggest issue I've run into with this is the Exchange Server Supportability Matrix. Always remember, that if something is not listed on that page, it is not supported. Many people think, if it's not listed, the Exchange PG just hasn't gotten around to updating the document. Thus, when there is no AD 2019 FFL/DFL listed, it kind of looks like they forgot and/or it's not supported. Well, since there is no 2019 FFL/DFL, there is no need to list it, and thus, it appears that it's not supported, yet it is, since 2016 is the current highest AD version.
So there you have it, hopefully clearing up some potential confusion with AD levels and Exchange version support.