T-Mobile says it's going to delay the CDMA shutdown from January 1, 2022 to March 31, 2022. T-Mobile's statement says they're doing it to protect the Dish's customers, since Dish has failed to do so. Personally, I'm betting T-Mobile is 'volunteering' to delay the shutdown in hopes it will avoid regulators forcing them to delay it by considerably longer than 3 months.
This probably buys Dish/Boost a little time in getting customers all moved over: a difficult task made more so by phone supply shortages. I don't think it will affect most MVNOs much, since they've already committed & moved on a timetable to migrate customers off the Sprint network. It might mean a few more months of legacy Sprint availability on Red Pocket, TextNow, etc.