@JW500, All Model 3's from 2018 onwards that have an Intel based MCU have the EXACT same hardware as "Legacy" S/X with MCU2 and Autopilot 2.5 and up. The only physical difference is that Model 3's hardware is liquid cooled vs. S/X which is air/fan cooled.
All Model 3's ever built have Live Sentry mode.
This feature is deliberately being withheld by Tesla for MCU2 equiped "Legacy" S and X's. Even the code for Live Sentry mode has been implemented and dormant since the 2022.16 firmware branch, which was released back in May 17th!!! [Code is still present on the 2022.36 branch].
But as with all things Tesla, it's impossible to know when or if it's even going to be enabled or not, just like the Apple Music firmware code that came and went and it never got enabled

.
As of today, the modems on all current Tesla vehicles sold are still 4G/LTE only.