There was a time you could just walk around with a radio receiver and spy on everyone's phonecalls (IMEI stingray). Iirc it's not possible/that easy anymore with LTE.
Can vouch that the first and second repos definitely worked as of about a year ago, but haven't personally tried since then. I don't have a USRP X310 (yet!) so can't personally speak to the fancy one.
Do/could SIM cards prevent this downgrade if 2G isn’t provided by your local provider anymore?
I know my Canadian SIM card somehow hides US providers from network scans, possibly with some geo or if/then rules (but visible from my EU SIM that tries its darnedest to latch onto the US networks and avoid the Canadian ones at all costs)