Deprecation states formally that the feature is no longer actively developed, and it serves as a warning that Microsoft may remove the feature in a future release. Removal isn't guaranteed, however; there are parts of the Win32 API that have been deprecated for 20 years but still haven't been removed. It's possible that Paint will continue to ship with Windows in a kind of zombie state: not subject to any active maintenance but kept around indefinitely since it's self-contained and not a security risk.
Indeed, the end of the development of Paint is not going to surprise anyone who actually uses the thing; the last time it received any non-negligible improvements was in Windows 7, when its user interface was updated to use a ribbon control. Before that, it had an interface that had been largely untouched since Windows 3.1. As such, Microsoft's official deprecation is merely confirming something that was already obvious; it's not an indicator that anything has actually changed.