OP is either from a different region than I was from, or a totally different generation.
a) They used to read a scripture describing the person's sin. They had some legal risk for this so they stopped. It was not better for anyone--the df person (probably often falsely convicted) got their reputation destroyed, and the congregation used it to do further damage to the reputation.
b) We always knew anyways because the elders' wives always told.
How 'bout this ? The whole IDEA of disfellowshipping is wrong in the first place.
Formally announcing it, saying, "Sister Slut fornicated with Brother Jones" would just have made it all worse. Why...