I’d say something like this - feel free to make changes…
Hi there,
You might already have heard of ClassicPress (https://www.classicpress.net), the 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. This means that as long as [PLUGIN NAME] remains compatible with WP 4.9.x, it will also be compatible with version 1.x of ClassicPress and users can continue to enjoy the benefits of your plugin.
However, planning is already underway for more changes in Version 2, and these changes will require plugin developers to make a serious committment to develop 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 keen user of [PLUGIN NAME] I would very much like to see it added to that list.
You probably know this already, but there is a handy condition that you can use in your plugin:
function_exists( 'classicpress_version' )
To make it easy for ClassicPress users to find your plugin you can add somewhere in the plugin’s readme.txt that your plugin is compatible with ClassicPress. Once you do, it will be very easy for people to search in the WP Plugins Directory on ClassicPress compatible plugins: Search Results for “classicpress” | WordPress.org
Looking forward to hearing from you,
Thank you very much,
[YOUR NAME]