It is theoretically possible though, that if you find a usecase for a protocol that happened to be not used for a hundred years, that you implement it and then use it. Not even the document about the protocol is necessary if you remember it well enough.

And a part of the self-responsible philosophy is also that "there is no implementation" is not even an excuse. As long as there are free programming languages and affordable computers (I.e. a raspberry pi) then you can very well create your own implementation. It might take years to even start due to the requirements to learn programming, but it's not impossible at all.

I meant: a communications protocol that no one else uses is as useful as one telephone.

More like a notebook. You could for instance use the email format to store your diary entries, or a todo list, or an index of useful links, etc. even if nobody else uses the standard anymore.