NMS is not supported API. It has never been and never will be. Whilst we recognise that some things possible with NMS are unlikely to be possible with the API, many things that NMS is used for are possible with the API, or would be possible with very trivial feature requests.
Before using NMS for any reason you should carefully consider whether you really need to use it or not.
Updates to NMS are a crucial aspect of being able to sustainably and reliably deliver new versions of Spigot and may occur without warning. As a developer using NMS you must ensure that your plugin gracefully handles unexpected situations.
The only guarantee given about NMS is that CraftMagicNumbers.getMappingsVersion() will be changed if any mappings changes are made (which could be frequently).