It is being released with the new identity upgrade. The API is helpful for linking external keys so it made sense to release these together.
Most API actions are free, but creating transactions requires funding an API key. Fees for creating transactions are based on the unsigned transaction size. For most transactions the fee is 4 sats/byte for the unsigned transaction size (usually about 500 sats per tx). For token transactions the fee is 8 sats/byte also based on the unsigned transaction size.
The API can also be used for getting info about users or to get lists of posts. You can also broadcast and listen for transactions once they have been signed.
Apps can also redirect to Memo to link and fund external accounts.
BSV API is not yet live but should be soon.