I'm curious to see what will happen with LTE-M in this context. Is it going to be phased out as well?

According to Ericsson: > Cat-M1 and NB-IoT are considered future-proof and are viewed as 5G technologies

But AFAIK Cat-M1 falls into the "LTE" umbrella, which means it could be phased out by T-Mobile

>The document goes on to mention that T-Mobile will retain a small 5 MHz-wide channel of LTE across the country. This will be maintained until at least 2035 to accommodate legacy IOT devices, which can range from ATMs to smart infotainment systems in vehicles.

Your old phones are safe for now

Pretty wild something called "Long Term Evolution" is being phased out