There was a thread a while back discussing this. Yes it seems like 300 2.8 and 500 f4 are going the way of the dodo. Which is a shame. The 500 f4 is so much smaller/lighter than the 600 that it's much more versatile. OTOH I personally have never found 300mm very useful for the wildlife that I shoot. I owned a 300 2.8 for a long time and it was my least used long lens. Hated to sell it. Such beautiful IQ. But it was collecting dust.I'm not sure....
Canon...hasn't released a 300/2.8 or 500/4 since 2011 or so. Two newer 400/2.8 and 600/4 versions have been released since that generation.
Sony...hasn't released 300/2.8 or 500/4 since A-mount. Only 400/2.8 and 600/4 in E-mount
Nikon...no 500/4 since 2015. 120-300/2.8 seemed to be the 300VR replacement in F-mount.
That seems to suggest to me that 300/2.8 and 500/4 sell a lot less than 400 and 600. Especially looking at Canon's history as I would have expected them to do MkIII DSLR versions and RF versions. But no.
I suspect the 500 f4 was favored by enthusiasts more than pros due to smaller/lighter/less expensive. And Canon/Nikon are clearly prioritizing the pro market. Maybe a combination of much improved optics in zooms and lighter weights of the 400 2.8/600 f4 are making the 300/500 unnecessary?
For my own needs the perfect combo is 100-400 zoom, 500 f4, and 800 PF(BTW I currently own none of the three). So in response to the OP, yes, it would be cool so see a Z mount 500 f4 w/built in TC. Not holding my breath... :(