We shouldn't add protocols for features where others can remain
oblivious to their presence or absence. For example, if some
directory caches start supporting a new header, and clients can
safely send that header without knowing whether the directory cache
will understand it, then a new protocol version is not required.
Because all relays currently on the network are 0.2.4.19 or later, we
can require 0.2.4.19, and use 0.2.4.19 as the minimal version so we
we don't need to do code archeology to determine which number
You did that thing where you start a sentence
Otherwise looks good, modulo a few typos that don't affect meaning.
Tim