The value of password_manager::prefs::kCredentialsEnableService controls whether the Password Manager will save credentials. When it is false, it doesn't ask if you want to save passwords but will continue to fill passwords.

This preference should also control autofill behavior for the passwords as in TODO comment here:
https://cs.chromium.org/chromium/src/components/password_manager/core/common/password_manager_pref_names.h?l=23&rcl=df46b528df23b652ae20473c318ed0568d66172a

Assigning to melandory@ based on the TODO comment. Please reassign if you are no longer an appropriate assignee.