Activations before May 7 get to keep the deal as long as you stay with the network.
It wasn't a lot, but it was something.
As of May 7, T-Mobile will no longer offer "free data for life" on new tablet activations. The promotion, which started in October 2013, gave buyers of new Apple iPads and other tablets 200 megabytes of free data a month as long as you owned and used your tablet with the carrier.
If you have a T-Mobile data-only tablet that was purchased on or before May 6 and activated the free data plan, you get to keep it. You do, however, need to keep an active data plan at a minimum to continue to receive this offer, which carries no additional charge if you're a voice customer (aside from a $10 line credit). Prepaid customers need to maintain an activated account, but you don't have to buy a data pass to use your free data.
More details are available on T-Mobile's support site.