I was one these longtime AT&T customers that jumped ship a month ago; I have several unlocked BlackBerry Key2 and essentially don't use data (roughly 1 GiB on any given month per unit over the past 2 years). The cost of these phones approach flagship territory, and there still isn't a suitable replacement on the market as integrated physical keypads go IMO, so it was a no-brainer.
That the much older KEYone and Priv models would continue to be supported on their network, yet the newer Key2 and Key2 LE models aren't on their whitelist despite supporting VoLTE and receiving an unusual number of system updates in the past year alone, was at first baffling. As I understand it now, the situation is a business decision (not tech constraint) to whitelist only older VoLTE-compatible models previously sold by AT&T---whereas the Key2 supposedly never was.
After all the periodic SMS reminders and dealing with their batshit annoying bot that would entitle itself to interrupting the first 30ish sec of my voice calls, I got really tired of it and burned a random Sunday afternoon porting over---saving over $300/yr per phone with still more than adequate data. Never looked back.
[1] https://www.att.com/support/article/wireless/KM1324171/
[2] https://www.att.com/idpassets/images/support/wireless/Device...