I also am unaware of any official guidance regarding allowable abbreviations. From anything I've seen, as long as the meaning of the abbreviation is clear in the context, I've never had an audit or anyone else restrict it's use. I do know for hospital records, most hospitals have a list of abbreviations that should never be used (particularly with medication names and dosing) as well as a list of approved abbreviations.
For example, in my practice, the clinicians use RA prior to a surgery description to indicate it is "robotic assisted" RA TLH BSO = robotic assisted total laparoscopic hysterectomy, bilateral salpingo-oophorectomy. For a rheumatology practice, it's going to mean rheumatoid arthritis. Hematology may use it for refractory anemia. It can definitely create confusion, or patient errors. But to my knowledge, for private practices, it is not prohibited.
I would also add with EHRs, it is usually easy to create user macros that if your clinician is tired of typing out an entire name, you can create a shortcut where the clinician enters just the abbreviation, but the medical record inserts the full name. But of course, that is in my own perfect world.