Twitter, in general, is a remarkably open company internally. Because it leaks things like a sieve and has people dig into its products for new stuff, this ends up being the case externally as well. Most major A/B tests are announced via the various official Twitter accounts, for example, and employees regularly tweet about or solicit feedback about new features. Editing tweets, for example, had a concerted effort to avoid it from leaking externally (specifically compiled out of builds rather than being behind a feature flag, codenames, documents mostly put behind “do not distribute internally”) and it was still a somewhat open secret, since you could stumble upon it in the codebase or when searching for bugs. Companies that “care” take far more extreme steps to hide things than Twitter does, or I would argue even knows how to do. It’s not in their DNA.

With that said, the timeline is somewhat secret sauce but mostly it’s just a glop of various parameters that aren’t very interesting to share. Like, how are you going to even describe it to someone? It’s easy to talk about changing a button color, but “we promote retweets 10% more now” is boring and not really something that most people care about.