T-Mobile's Uncarrier 8.0 announcement was centered around letting you keep your unused data for a bit longer. Data Stash is already available on all Simple Choice post-paid plans, but now it's coming to prepaid too. Starting on March 22nd, people on a prepaid Simple Choice plan from T-Mobile will have Data Stash.
This only covers customers that are using T-Mobile's in-house prepaid service, not any MVNOs that happen to run on T-Mobile's network. Additionally, you won't get Data Stash if you're on a non-standard plan like the semi-secret $30 5GB/100 minute deal.
The data you don't use each month will be dumped into your personal Data Stash for use when and if you exceed your data allotment in a future billing cycle. Like postpaid customers, qualifying prepaid users will be given 10GB of LTE to get started. After that is gone, unused monthly data will start rolling over.