Back when IBM provided support for Domino it was supported on IBM i 7.4 as per
https://www-01.ibm.com/common/ssi/ShowDoc.wss?docURL=/common/ssi/rep_ca/4/897/ENUS219-214/index.html&request_locale=en
Now HCL is trying to tell me that Domino support for IBM i 7.4 is only in testing:
https://hclpnpsupport.hcltech.com/csm?id=kb_article&sysparm_article=KB0073794
Does it typically take a year or more for HCL to support new operating systems with their products? I didn't see Windows Server 2019 listed either.
I know that the IBM i people offer early ship to many vendors of new releases. I suspect Windows does also. Does HCL not participate in such programs?