I browsed through it a bit and these are some details that raised questions or which I found interesting:
There's multiple mentions of slop, for example: SlopsAuthorScoreFeature in HomeTweetTypePredicates. That means everyone gets a slop score between 0 and 1, which makes me wish that it was openly visible and that people with a high slop score would get a little piggy emoji next to their name.
There's a CLIENT_TWEET_TAKE_SCREENSHOT action, which is likely used to keep track of when a (mobile, presumably) client takes a screenshot. I hadn't considered this before, but for a social media app where posts are often shared externally through screenshots, keeping track of this can give you another engagement metric.
They have two types of NSFW filters: isNsfw and isSoftNsfw, but I couldn't figure out the distinction. Other metadata types include: isGore, isViolent, isSpam, isLowQuality, isOcr.
In ContentFeatureAdapter there's a getTweetLengthType function which shows the range for each tweet type. This is used to set TWEET_LENGTH_TYPE elsewhere. I wonder if it would help your virality to switch up your tweet lengths to regularly put out tweets which hit every length category, or if it doesn't significantly affect your potential reach.
There's a hardcoded list of top-level Grok topics [0]. Just mildly interesting to see what they consider to be top-level categories. Anime has achieved a significant cultural victory by getting separated into its own major category.
The timeout values for different service request types varied a lot across the application, which makes me curious about how they settled on those numbers. This is a question I've pondered in the past but haven't gotten around to researching deeply.
[0] https://github.com/twitter/the-algorithm/blob/c54bec0d4e029f...