Although this issue is specific for PBTS, I think it is worth a more comprehensive discussion.

On the one hand, we want to keep track of changes in the specification, keep its history, so that anyone is able to follow the process that lead to it. In the specific case of PBTS, this includes the previous solution, BFTTime, as well the first proposal for implementing PBTS.

On the other hand, this repository, specially the content under spec/, should ideally contain only updated information, that reflects the current state of the implementation. So, ideally, both BFTTime and the first proposal of implementation of PBTS should not be present in the master branch.

Notice that old versions of the documentation, as long as the implementation, are always available in branches. For instance, the BFTTime documentation will remain available in v0.35.x branch, as it was the solution adopted in that version of Tendermint. The same is not true for the first proposal for implementing PBTS, which however could be maintained in some other branch.