According to this, you can have both SFP 15 and 16: If there are issues with Win 11/ SFP-15 , you will get the SFP-16 upgrade too, as soon when it is published.
Apparently, Sound Forge Pro 15 was released prior to Windows 11 release, so the specs didn't list it being compatible.... but, Magix released this page upon Windows 11 release, and shows the then current Sound Forge Pro 15 being listed.... SFP16 wouldn't come out for another 6 months after Win 11 if I'm not mistaken.... So, YES, it apparently was tested and claimed compatible! Good to know as I was wondering the same thing recently.