When disabling SMT in BIOS, S3 (Energy Save) is also not available anymore on Windows 10. If checking with powercfg/a S3 is not available.
For some application disabling SMT is necessary because of performance benefits (that's also the case for Intel's HT)
S3 disable with SMT disable is not only the case on my motherboard, but also on some others I've seen so far; also some internet research shows that behavior.
Why this happens I don't see a relation, please fix if possible.
If you are using a Ryzen or Threadripper CPU this is expected behaviour. If you wish to use Sleep mode, you cannot disable cores/threads.