RTSP is indeed the predominant protocol used by IP CCTV cameras - that is what might be regarded as professional grade CCTV equipment. However it seems consumer or ‘smart home’ cameras more typically use HLS that is HTTPS Live Streaming instead.
For example Netatmo use HLS on their indoor, outdoor and smart doorbell cameras. I believe NetGear Arlo might support both, I have seen a comment saying it (now) supports RTSP but that stream is encrypted. The fact NetGear Arlo now supports HomeKit suggests it also supports HLS since that is what HomeKit supports.
So support for both protocols would be preferable.
For what its worth the app ‘SecuritySpy’ for Mac only supports RTSP and not HLS. 
Also it seems older or simpler cameras may still only support MJPEG which as far as I am aware does not support audio.
I believe the Ring doorbells support SIP and the Ring Stickup Camera uses MJPEG.