Lowest plan:
"In times of congestion, your data may be temporarily slower than other traffic."
Upper plans:
"Get access to 50GB of 4G LTE premium data per month. ... . In times of congestion, your data may be temporarily slower than other traffic after exceeding 50GB/mo/line."
Of course, that's 4G, not 5G, but before they tell you whether or at what point they throttle 5G, I can't say what "having unlimited 5G" means.
For context, it looks like this means what a commenter on a Verizon forum says (quoting verbatim, looks like the GB numbers changed):
"Verizon has 3 levels of unlimited data plans. The cheapest one has the possibility of being "throttled" AT ANY TIME there is congestion on the local towers. The mid-tier has no throttling until you have used at least 22 GB in a month on a line. The highest tier has no throttling until you have used at least 75 GB in a month on a line."
But...they are telling you? It's right there on each of their plan pages.
* Verizon: $80/month plus taxes for one line with 5G on Play More Unlimited - https://www.verizon.com/plans/unlimited/
* T-Mobile: $60/month plus taxes for one line with 5G on Magenta or $70/month with taxes included on Magenta Plus - https://www.t-mobile.com/cell-phone-plans?lines=1
* AT&T : $65/month plus taxes for one line with 5G - https://www.att.com/plans/wireless/