Thanks ohrenkino, for this information:
It is not unofficial - it is an official standard compliant user-defined field. ID3V2.3 allows user-defined fields.
For other programs that recognize the BPM TAG (BPM), I don't know...
Adobe Audition, who are Leaders in the field, recognize this Tag, I conclude that it opens the door to other Software companies, perhaps less known, to recognize this Tag
Correct if I am wrong, I believe that the kid3 program recognizes this Tag...