UPDATE 2/12/21
The Exchange Sustained Engineering Team continued servicing all customer issues for Exchange 2016 that were opened with Microsoft by the end of Mainstream Support, which was Oct 13, ...
Updated Feb 12, 2021
Version 12.0
Blog Post
I believe the statement made earlier is incorrect regarding the use rights of Hybrid licence key for mail relay and user management automation scripts.
The guidance has always been that the Hybrid licence key rights are fine so long as there are no Mailbox workloads hosted on the server.
This statement is made in the licensing FAQ and the guidance has also been updated to reflect the fact that 2019 servers are not eligible, so it appears to be an active guidance.
Refer here: https://www.microsoft.com/en-us/microsoft-365/exchange/microsoft-exchange-licensing-faq-email-for-business
"If you do not host any mailboxes on the servers used to connect to Microsoft 365 you can license them using the Microsoft 365 Hybrid Configuration Wizard (HCW) which you can find here. The HCW validates your Microsoft 365 subscription and installs the appropriate licenses on your servers. Note that the free Exchange Server license is not available for Exchange 2019 hybrid servers."
It is also worthwhile checking with your licence agreement provider, as you may already be entitled to run Exchange Server on-premises under some M365 licensing agreements.
Although I know this will not be of comfort to many smaller seat customers, if eligible it will provide a mail relaying solution in the future, if the Hybrid use licence becomes deprecated.