Spigot 1.15.1 support?
Yes, I am that guy, asking the obvious perhaps.
We're going through a list of API that we use and this is the last one at the moment that we're not certain about if it has support for Spigot 1.15.1. From what I (try to) understand this has NMS stuff in it, so it's likely this has changed between 1.14 and 1.15.
From looking at it, it does contain NMS but it will continue to support new versions as long as the class structure does not change. If you look at the main class, it contains a variable called nmsver which contains the detected version of your server. It then uses this variable to lookup the required classes to show an ActionBar, so to me it looks future-proof.