[Quoth]

The Amazon announcement suggests only WiFi devices after Download and Transfer ends.

My pointless Kindle DXG had the wrong time and date after battery was flat last time. However just now I turned on Mobile as usual got "EDGE", the high speed version of 2G nearly as fast as original 3G (The 3G later went about x10 faster if the cell wasn't busy and there was HSPA or similar upgrades).

Anyway, after a while online the time & date fixed and I tried Sync. Nothing changed.
So I went to My Content & Devices and "sent" an ebook explicitly to the DXG. Amazingly then the Sync on the DXG downloaded it.

The 2G / EDGE survives in some European places that don't now have 3G (though some may still have 3G) because of smart meters, traffic lights, or alarms that use GSM 2G/3G and because of Cell Breathing and other issues, the 2G (basic 14.4 kbps, EDGE 245 kbps) was always the reliable fall-back. That would have been rare in USA.

Obviously it's only old Mobi/KF7, but Calibre can Add it. While mobi is rubbish compared to azw3 / epub, it's OK for a novel that has all one body text (more than 90% of novels) and simple headings.

I'll check again once the D&T ends, maybe early March. The DXG battery was replaced a couple of years ago and mysteriously (unlike the K3) is still like new.

Edit:
It suggests either Amazon bought a bulk forever account for the SIMs and mobile/cell service, or they are currently still paying the local Irish mobile service provide for traffic at least. Perhaps the SIM accounts will end on the 26th of February 2025, though setting Time & Date probably still works as an emergency call works on Mobile Phone with no valid account.