After the allowance is used up, service pauses or slows under that plan's own rule. Either can happen. It depends on the plan you bought, not on one site-wide next step. This matches the FAQ answer about running out of data.
There is no single slowdown speed
Plan data has no field for "how slow it gets after the allowance". This article cannot give you a number. If a plan name says it is speed-limited, that limit belongs to that option itself. It is not a second speed that starts after the allowance. Do not copy one plan's label onto another plan.
If you still need data
You can buy another plan for the same destination from your account. Details from the earlier order are reused, so you do not fill them in again. Plans are on the destination list. Pick the place you still need, and read the allowance on that page.