According to a report from the firm Wave7 Research, June 1st will be the last day new Boost Mobile customers can activate on the Sprint network. After that date, existing customers only will be able to continue to use their current device on their current plan on the Sprint network. So if the Sprint network still provides the best coverage for you in your area, you may want to join Boost Mobile by June 1st. I suspect similar deadlines and stipulations will be put in place for other Sprint MVNOs. Tello Mobile, which was previously a Sprint only MVNO has said that it plans to support Sprint service at least until June of 2021. So that seems to line up with Boost's timeline as well. However, Allvoi recently told me they plan to continue to have access to the Sprint network until the end of the year.
Also stated in the report from Wave7 Research is that currently, activations are allowed on the Sprint network by exception only. This will continue as long as Boost Mobile has legacy devices to sell that only support Sprint's network. Only a small number of activations are said to be on Sprint.
Boost Mobile does continue to highlight its Sprint-based plans on its website. However, with the exception of a Walmart exclusive plan for $35/month with 6GB of high-speed data, all of their Sprint-based plans mirror their T-Mobile "Expanded Data Network" plans. Boost's legacy Sprint plans are no longer listed as being available to new customers.
The upcoming changes reported here are not unexpected. However, they are still bound to hurt some customers. I do occasionally see people in online forums state that the Sprint network still works best in their areas. Indeed, Jeff Moore, Principal of Wave7 Research tells me that Boost previously abandoned their policy of not allowing customers to port back over to Sprint if they are unhappy with the Expanded Network for that very reason. Boost does not want to lose customers in those areas. But perhaps this policy will change again after 6/1/21. Current customers on Boost with the Sprint network should go back and reread the first paragraph of the article. From what I can tell, it seems that after 6/1/21, if a current customer changes their plan or upgrades their device, they'll no longer have access to the Sprint network. So if you plan to keep your Boost Sprint plan for the long haul, be sure you're happy with your device and plan heading into that 6/1/21 date.
"Not being able to activate on the legacy Sprint network is a minor negative for Boost. There are still areas where the legacy Sprint network has better coverage. That said, I get the logic, as the Sprint network has one foot in the grave." --Jeff Moore