Jetstream isn't an official change to the Protocol, it's an optimization I made for my own services that I realized a lot of other devs would appreciate. The major driving force behind it was both the bandwidth savings but also making the Firehose a lot easier to use for devs that aren't familiar with AT Proto and MSTs. Jetstream is a much more approachable way for people to dip their toe into my favorite part of AT Proto: the public event stream.