Seems the HMC5883L stock is worse than MPU6000 / MPU6050 / MPU9250 which are all EOL/NRND for years now, yet FC boards that use them are still being built. I thought that for instance digikey did not have any of these years ago. Now I see that they do have some (MPUxxxx).
I recall a year or more ago that a developer thought to make the LibrePilot mag driver automatically recognize HMC5883L or QMC5883, but it never happened. I don't see any git log comments in next or tracking about this. It is possible that it is done in a private branch somewhere though.
I suspect that most people who want mag now use the DJI/Naza GPS/mag which is supported by LibrePilot. This GPS/mag uses one port for both GPS and mag. That leaves more ports free for other things like OSD. Also, with I2C mag (HMC5883L) there are sometimes issues with I2C bus termination (pullups) requiring users to solder some stronger pullup resistors. The DJI/Naza GPS/mag also appears to be EOL since 2016, but are still available.