Why is there no ‘hybrid key’ for Exchange 2019?
Because Exchange 2019 is downloaded AND licensed through the volume licensing center, the Hybrid Configuration Wizard (HCW) does not have the ability to license or provide a key for an Exchange 2019 ‘hybrid server’. Simply use Exchange 2016 instead, and HCW will happily license it for you
I downloaded EX2019CU10 (aka "latest" as of today) from Microsoft public website, installed it, and can't license the server with HCW now.
I did the same for EX2013, EX2016 (whenever "management server" functions were required for hybrid), it always worked and was supported flawlessly.
Why is EX2019 so special? Do you suggest staying with EX2016 till EOL or buying a full Exchange 2019 Std license just to add a proxy address alias for the synced/federated user?