This is going back to that “cap-in-hand” approach I was trying to avoid. But I agree that it should be simplified … how about something like this (again… feel free to edit)…
I am a keen user of your plugin, [PLUGIN NAME], and I have recently decided to migrate to ClassicPress (classicpress.net). This is a hard fork of WordPress 4.9.8. It’s in Beta 2 now and the team is working hard on the first production release, ClassicPress 1.0.0.
Version 1 of ClassicPress will always stay backward compatible with WP 4.9.x. and, at the moment, [PLUGIN NAME] is still working well for me. In fact, as long as it remains compatible with WP 4.9.x, it should also be fine with version 1.x of ClassicPress.
However, planning is already underway for more changes in Version 2, and these changes may require plugin developers to make more of a commitment to developing for ClassicPress. There are already a number of developers indicating their willingness to do so, and ClassicPress users are now compiling a list of ClassicPress-friendly plugins which are quickly becoming their preferred options. Obviously this presents a good opportunity for developers to gain access to a whole new market, and as a satisfied user of [PLUGIN NAME] I would very much like to to be able to continue using your product, rather than look for an alternative.
If you need more details about the techical requirements for ClassicPress you can find more information here … [LINKS]
Looking forward to hearing from you,
Thank you very much,