We had to do some more testing then expected but we are planning to move to PHP 7.4 from 7.2. The only downside is PHP has had some removals from 7.2 to 7.4 and some older scripts sites have might break. As a result and due to a few things failing in testing we are going to update N7 and N8 first near the end of January and the others a few months after.
We posted this in the verified account slack last week and are posting here after some requests on Twitter.
Wordpress is fully supporting 7.2 and the messages are more of a reminder. PHP 7.2 is supported for another year but we will be moved to 7.4 in the next 2-3 months at the latest.