here comes this (evil?) PR

Interestingly, 666 is a positive/luckily number in Chinese, expressing "good" and "proficient": https://en.wikipedia.org/wiki/666_(number).

The implementation is not complete. After running remove_tokens_for_client(), the ghost of the service principal still lingers in AppMetadata:

{
    "AccessToken": {},
    "AppMetadata": {
        "appmetadata-login.microsoftonline.com-a7003d8c-e50f-4371-91a6-ef37bba4ab23": {
            "client_id": "a7003d8c-e50f-4371-91a6-ef37bba4ab23",
            "environment": "login.microsoftonline.com"
        }
    }
}

What if I lose track of service principals client IDs? How do I purge all service principals' access tokens, while keeping users' access tokens?

What if I lose track of service principals client IDs? How do I purge all service principals' access tokens, ...?

Hmm, MSAL's ClientApplication always requires client_id as its required parameter. This question sounds as strange as "what if I forgot my local Windows username, how do I purge all users' content from the current PC?". Is there a legit scenario for that? That being said, I suppose you can do format c: or its equivalent rm ~/.azure/msal_token_cache.json if desirable.

How do I purge all service principals' access tokens, while keeping users' access tokens?

You would have to create different ClientApplication objects with different token cache persistence files, so that you may be able to delete some of those files.

The implementation is not complete. After running remove_tokens_for_client(), the ghost of the service principal still lingers in AppMetadata:

{
    "AccessToken": {},
    "AppMetadata": {
        "appmetadata-login.microsoftonline.com-a7003d8c-e50f-4371-91a6-ef37bba4ab23": {
            "client_id": "a7003d8c-e50f-4371-91a6-ef37bba4ab23",
            "environment": "login.microsoftonline.com"
        }
    }
}

The residue of AppMetadata is technically not an incomplete implementation. The AppMetadata was meant to be persisted forever, because (1) they would be used to remember whether an app belongs to a "family" (although that characteristic does not necessarily apply to service principal); (2) they contain no sensitive tokens anyway.

This question sounds as strange as "what if I forgot my local Windows username, how do I purge all users' content from the current PC?".

This is possible for user accounts logged into MSAL as users can be retrieved with ClientApplication.get_accounts():

Azure CLI has a logout_all_users() which removes all users, but logout_service_principal() is currently incomplete due to no equivalent of ClientApplication.get_accounts().

although that characteristic does not necessarily apply to service principal

I agree the client ID for user authentication (such as Azure CLI's client ID) is not sensitive, but some users may treat service principal's client ID as sensitive date and want it to be "obliviated".