I'm not even sure it has to be registered. In at least one court case I know of (a local landlord case) the court assumed that a mailed letter was delivered properly.
There is no guessing involved. That is how you notify a company.
I believe you are confused about what I am saying. I’m not saying that google will change anything just because you sent a registered letter. You send a notice, and they might change things or they might not. The notice in and of itself does not compel them to do anything.
What power does such a notice have then? In a court case you can say “your honour, we sent notice on day x, by registered mail, here is our retained copy of the content of the letter”. And the courts generaly assume that such notices were received and read (consult with your lawyer about any possible edge cases, of course). This is not generaly true if you send your notice via carrier pigeon, or shout it at their air vents, or stuff it in a teddy bear and flush it down the toilet. Those are less legitimate channels in the eye of the law.
So what then? They received your notice and you can prove it. What will that change? Maybe nothing. Maybe everything. If you have some legally colorable argument, it can help you paint their action or inaction following the notice as willfull. In some circumstances that increases damages, or makes them liable.
In practice what it wins you is that it will be read by some lawyer kind of person, and if that person reads it and says “uhh, this person could sue us and that would be bad” then they usually have the clout to change things. That of course depends on what you are giving them notice about. If you wrote some rambling with no actionable ask and no chance of a succesfull lawsuite they will ignore it. They probably get plenty of those.
