Windows 11 comes with only 64-bit and cannot support 16-bit. The only versions that support 16-bit are some Windows 32-bit versions.

I have the pleasure of supporting an ancient 16 bit program for a client. It worked fine on their old server running Windows Server 2003 (32 bit) but will not work now that a new 2008 R2 server ...

Assuming the software is designed correctly, porting form 32-bit to 64-bit should be simple. Simple like you don't have to change anything. 32-bit programs would just ignore half of each register ...