They're not sending out 100BASE-TX ONTs to lower-tier subscribers. You're all using the same hardware; they're just limiting the packet rate to match the plan.
Regardless, the latency difference between 100BASE-TX and 1000BASE‑T pales in comparison to the difference in modem speeds between different providers' network types.
Fiber providers with an ONT get LAN-like speeds, while cable providers' DOCSIS modems add tens of milliseconds of latency, LTE modems are similar to double that of DOCSIS, and satellite providers range from similar to LTE all the way up to hundreds of milliseconds of latency, depending on the orbit and if there are ground stations nearby.
Again, within any provider, you get the same latency with any the plan, but changing providers can have order-of-magnitude differences.
Regardless, the latency difference between 100BASE-TX and 1000BASE‑T pales in comparison to the difference in modem speeds between different providers' network types.
Fiber providers with an ONT get LAN-like speeds, while cable providers' DOCSIS modems add tens of milliseconds of latency, LTE modems are similar to double that of DOCSIS, and satellite providers range from similar to LTE all the way up to hundreds of milliseconds of latency, depending on the orbit and if there are ground stations nearby.
Again, within any provider, you get the same latency with any the plan, but changing providers can have order-of-magnitude differences.