DEPRECATION.md states

Beware that users will often be using functions, classes and methods that we,
the Qiskit developers, may consider internal or not widely used. Do not make
assumptions that "this is buried, so nobody will be using it"; if it is public,
it is subject to the policy. The only exceptions here are functions and modules
that are explicitly internal, i.e. those whose names begin with a leading
underscore (_).

While I think the "if it is public it is subject to the policy" part saves the passage from being wrong, I think the passage reads oddly. It predates the "What is the public interface?" section and was written, I think, when "public" meant "anything not starting with a _." I think this would be more in keeping with the current policy:

Beware that users will often be using functions, classes and methods that we,
the Qiskit developers, may consider unused. Do not make
assumptions that "this is buried, so nobody will be using it"; if it is public,
it is subject to the policy.

I removed "internal" because if Qiskit developers consider something internal it would not be documented and thus would not be public and subject to the policy. Likewise, I removed the last sentence since I think it corresponds to the older definition of public.

I was puzzling over this section because qiskit-ibm-runtime copied this file before the "What is the public interface?" addition, so I was trying to decide if I could remove a function not starting with a _ there or not.