Honestly, that seems like a lot of unnecessary complexity, I'd rather go with either always or never including it.

What does an implementation have to do if we don't include sequence number and feed id in both the hash-encoding and the transport encoding? The simplest scheme that comes to my mind (and that incidentally would require little changes to sbot) is to look up in the database for the previous message. The db stores the feed id and sequence number of the previous message, so we get them. Add the feed-id to the metadata, increment the sequence number and add it to the metadata, then write the message with this additional information to the database.

You probably want to add in some caching for performance, but effectively all this does is a reduction.

Whoever is crazy enough to reimplement ssb, they will:

• implement a handshake and a streaming cypher
• implement a multiplexing system and its codec
• decode all the multiformats
• set up a database
• write a gossip scheduler
• recognize and produce a bunch of rpcs

I don't think adding a reduce operation to that list will make a relevant difference in development effort.