I got a response from NAG:
Thank you for your query in GitHub. That repository is not currently active and will be removed to avoid confusion.
Unfortunately, using the NAG Compiler on GitHub runners would require a major change to our licensing software.
Please be assured that we do not view this issue as unimportant, I am sorry that our position was miscommunicated to you in this way. We are actively considering alternative options for licensing but are not yet in a position to implement the necessary changes.
Although it is still unclear what will happen and when anything will happen if ever, it seems that we have made some progress compared with the previous response:
our developers do not view this item as important where our product(s) is concerned.
So it is not hopeless now. I hope I didn’t need to bring this problem to the community (in addition to communicating with NAG), but publicity did help.