End of July 2017, Microsoft published a list of features those are going to be deprecated in the future version of Windows 10. The classic 'Microsoft Paint' (mspaint), which was available in all versions of Windows, was also part of that deprecated list.
Along with 'Windows 10 Insiders Preview Build 17306', Microsoft started giving an alert message saying that the product will soon be replaced by the 'Paint 3D' app.