T-Mobile sabotaging old phones?

Sorry for the click-baity post title, but it’s quite a serious question.

I have been having on-going issues since last year now, specifically curiously since the last android update I received via T-Mobile where I get dramatic issues 60+% of the time, where everyone talking to me on the phone sounds digitally garbled and time-delayed even though I sound completely fine to them.

I of course have been through many many fix attempts with T-Mobile to which just finally came to the convenient “You’ll have to buy a new phone” fix as my S8+ is technically “End of life”.

Now, I figured before I went through and bought a new phone (as I would be in need of a decent one capable of doing long video and heavy tethering without overheating) I’d literally try every other thing I can think of, one thing of which curiously was setting the network mode to LTE/3G instead of LTE/3G/2G.

Incredibly I've not had an issue in days, so this peaked my curiosity.  Was my phone using 2G for some reason despite being in an entirely LTE/4G area (and even showing the 4G icon)?.  But why would it possibly be doing so?  Then I recalled the odd update I received from T-Mobile right around when this started, which did nothing to satisfy my curiosity when I noticed that according to my android information my phone is of a "December 1st, 2020" security update level, oddly at that since at minimum my S8+ was still within support cycle until May 2021 and even at that Samsung still released official security updates to it up until November 2021.  So what was this curious update that T-Mobile gave my phone just oddly before these issues started?  I cannot find any public information from T-Mobile as to the updates they do, let alone what this update was and why it apparently was not a security update to android itself.

Has anyone else been experiencing curious call quality issues on older devices that just seemingly started shortly after their phone may have reached a support EoL?

Would anyone from T-Mobile kindly be able to give me at least some reasoning to the software version discrepancies and oddly undocumented updates?  Or at least some technical reasoning as to what possibly on a just-year-old EoL'd phone would even remotely cause these kind of issues, and only on incoming audio, not outgoing?

I mean if it wasn’t for being at T-Mobile as long as I have been and them having really nice customer service people I would have probably long long jumped ship cause paying service over a year for literally less than 40% service reliability is a bit insane, but my hopes of them resolving this have largely disappeared now as my issue seems to be getting worse now.