Looking at the specific example of Contact Form 7, I am using version 5.1.6 of this plugin on a couple of ClassicPress sites and it is working fine. The current version is 5.2.1 which only supports WP 5.3 or higher.
Here are a few ways we could handle this, again using this plugin as an example:
- Reach out to the developer and ask them to either restore WP 4.9.x compatibility or continue to officially support the 5.1.x versions.
- List the latest 5.1.x version of Contact Form 7 in our directory, changing the minimum necessary to make it clear that this is supported by the CP community.
- Fork the 5.1.x version entirely and give it a new name.
- Wait for the CP community to develop alternatives.
Or some combination of the above.
If there are no features in the 5.2.x version of this plugin that you need on your sites then currently the easiest thing to do is to install 5.1.x and ignore the update notices. There are also plugins and code snippets that you can install to hide the update notices altogether. This is not ideal but it will work as a temporary solution.