I need to learn what those do, the link stuff. Sounds important.

I can't use GPS:

1. Our local ATC gives permission to fly but without ANYTHING being fitted that is more than basic gyro/accelerometer.
2. A fair bit of my flying is low level with overhead at 2/3 metres above the craft.

The reason I question SNR is I was a Tester for FrSky on the X12S programme (for six months). The summation of worldwide tests indicated it wasn't the panacea people thought. The only use established then (a while back admittedly) was it might be minimal help for extremely high noise environments, actually places that are in most countries now neither sensible nor legal to fly. That was fixed wing related, BTW, not that legality has improved for any UAV.

Due to where I fly I fortunately never had to deal with the hassle of having to contact ATC, but this is the first time I ever heard of ATC dictating what electronics you could run on a UAV. GPS is just a receiver and not a transmitter so there should be no risk of interference with any ATC systems. The only thing I can possibly think of is that they're worried about a UAV flying itself on a GPS dictated route of it's own accord (maybe into an active manned flight path) with no manual control by the pilot, but plenty of DJI camera drone operators seem to get ATC clearance and those have an abundance of autonomous flight electronics onboard including RTH which can't be switched off.

You might be right about the SNR. There were a number of YouTube videos published highlighting it's importance but it seems like that might have just been some of the "influencers" not really understanding what they were talking about. It does however seem odd that the Betaflight developers added that as a new dedicated OSD element if it's not a lot of use, but the ExpressLRS developers seem to agree with what you have said (see HERE). Maybe I will remove RSNR from my OSD and have one less thing to unnecessarily worry / alarm me while flying. So only LQ and RSSI dBm values are the RC link health values which seem to matter.

Yes, ATC insisted on NO autonomy, we only got permission (Memorandum of Understanding) on the basis that what we flew was pilot controlled at all times aka a model aircraft and would not fly away. Also we have a 300' foot limit for planes and a 150' limit for quads and helicopters due their single ended ILS requiring low half circuit if easterly wind.

Betaflight (and I cannot express how much I admire and appreciate the Devs effort) has IMO long reached very stable maturity and its "polishing the diamond" or dealing with new differing hardware now.