Sorry to be dismissive but if people need a reference PDS to implement a protocol, I kinda lose faith in its expressiveness.
I'm contrasting this through the prism of having seen people implementing ActivityPub software as early as 2017, when it was merely a draft and everyone was complaining about how many things had been left out.
There are dozens of things being built on the protocol. Just not PDSes. And it’s not because it’s difficult, but because there’s no point until there are others to try and connect to.
Sorry, I feel like I'm not getting my point across: if building independent software that implements a "federated" protocol doesn't give you a "PDS" then the protocol has failed. You shouldn't need a reference implementation to do it, you should be able to federate with multiple instances of your own software or with other independent implementers.
