Any default microphone preference setting to change voice for anonymity? Is there any default microphone preference setting to change voice for anonymity?
If no what are the popular free app for Mac (Catalina)?
A Mac can change its own voice, like when it's reading a text file, but not your voice.  One free software package that does what you want is MorphVox Jr. It can change a man's voice to a woman's and woman's to man.  There are examples on the link I sent. This version is free. The full version is $40.  

This link lists 5 top voice changing apps. They are Voxal Voice Changer, MorphVox, Voice Candy, Voice Changer Plus, and Voice Swap. You can read about them on the link.