The class io.narayana.lra.client.NarayanaLRAClient was created to simplify the narayana implementation of the MP-LRA specification. We did try to include a Java API in the spec and this class would have served as a PoC, but we had to remove it from the spec because we were accused of being overly ambitious and no further work, beyond our own needs, was done on the class.
The reason we don't want to support it is that it is untested in a general usage scenario and was evolved specifically for use by the implementation. If we don't mark it as deprecated then users may get the undesirable impression that it is supported.
Note that there are a number of warning in the javadoc for this class indicating that it is not for general usage but making it explicit will help because users don't always read the javadocs.