This is mainly to prevent TSC members (people responsible of the technical direction of powsybl) ignoring what the point is of some repositories - a situation that can happen when repositories are created during  night with no warning. Since we are only 8 members, if we agree that we need the majority to have consent, it means 4 votes which is not so much... If in the future we are more and/or it takes too much time to validate a change, we can rethink that, for now I don't think the constraint is very significant. It really doesn't need to be formal, it's just to keep TSC members informed with a minimum delay.
Updating MAINTAINERS file and sending an information email is really just to keep transparency with powsybl's contributors and users.