5G was developed with the idea of spectrum sharing. If the control channel runs in a 4G compatible mode, both 4G and 5G devices can attach and individual timeslots (or whatever the correct term is, cause it's not TDMA) will be encoded differently. Maybe it's a little less spectrum efficient on the control channel, but it makes it more likely to keep 4g devices online than in past updates where a whole block had to be 2g, 3g, or 4g.

Hopefully car makers learned from the 2g/3g shutdown to at least have a way for the (eventually) obsolete modems to not drain your battery when there's nothing to talk to. But then, my 2013 ford shipped with a 2g modem, replaced under recall with a 3g modem, and last I checked doesn't have a viable 4g modem (connected features weren't very useful anyway).