Cool to know. So as an Earth standard, it was probably adapted by Betazed for use in non-verbal, non-telepathic communication.
BSL got legally recognised in the UK as an official language in 2022 (alongside English and Welsh). That status creates a whole lot of legal mandates for accessibility and use, and may grow the language in future years. About 2/3rds of speakers identify it as their first language, the remainder are either hearing users, or deaf users who have other primary means (spoken word, lipreading, auditory amplification etc) they use toutinely.