According to https://github.com/dotnet/designs/blob/master/accepted/2020/better-obsoletion/obsoletions-in-net5.md, .NET 5 will obsolete the UTF-7 encoding:

The UTF-7 encoding is insecure and should not be used. Consider using UTF-8 instead.

• DiagnosticId: SYSLIB0001
• APIs:
  • System.Text.UTF7Encoding (constructors)
  • System.Text.Encoding.UTF7