A while ago I contact Yoast through their website to ask if they’d be supporting ClassicPress. This is the answer I received.
First off, thank you for reaching out and sorry for the delayed response.
Secondly, at Yoast we maintain the policy of only supporting the current and previous version of WordPress (so 4.9.x and 5.0.x as of now). This also means that once 5.1 is released, we will stop supporting 4.9.x., but we will keep supporting the Classic Editor plugin.