When T-Mobile moved to unlimited with the ONE plans, they gave You "unlimited" tethering at "3G speeds", which turned out to be 0.5Mbit/s, an unusably slow speed in 2018.
The Magenta plans gave you 5GB-50GB of full-speed tethering before dropping you to "3G speeds". The current Go5G plans are similar, with a limited amount of usable tethering data before you're, for all practical uses, cut off.
Before the ONE plans, there technically was no hotspot usage limit, but since you had a limited amount of high-speed data, your hotspot was effectively limited to whatever your plan gave you.
All the US carriers limit hotspot usage, partly to prevent someone hooking up a computer to download 50TB of pirated movies while clogging up the bandwidth for everyone else on that tower, and (moreso) partly because they're greedy.