By December 31, 2022, 3G will be shut down on all 3 major phone carriers. AT&T has already stopped 3G services on their network. Here's an article about the timeline of all the networks shutting down 3G.
What is 3G?
Any device that relies on 3G will not be supported and may just run only on wifi. This makes cars and 3G complicated. You can stll drive your car, but if certain devices like remote start used 3G technology, it may not be supported anymore.
There are options that are affordable to upgrade to a newer phone. If you're like me, I keep all of my old phones and I just have a new phone that is current. Check out this article from CNET about Apple's cheapest phones. You don't need to spend $1000 to keep up to date with the technology.