They can and they do. TMo will discriminate service levels based on pricing plan.
Cheap plan:
During congestion, customers on this plan may notice speeds lower than other customers and further reduction if using >50GB/mo., due to data prioritization.
Middle tier:
During congestion, customers on this plan using >100GB/mo. may notice reduced speeds until next bill cycle due to data prioritization
So at the low end, you’re deprioritized at any time. In the middle, only if you’re over a monthly allowance.
Mint, which uses their network, gets lower than all native customers
Just know that deprioritization is a thing. Mint Mobile piggybacks off of the T-Mobile network. So even with your tasty unlimited data, you might see your speeds go from running to crawling because T-Mobile’s customers are the priority, not you. It’s one of the unfortunate drawbacks of Mint’s otherwise clever business model.