Twitter contributed a lot to Map-Reduce, ETL and Scala communities: IMO they punched above thier weight.

Sadly, I think their best open-source contribution days are behind them with all the hardcore engineering they now have to do with fewer engineers.

Edit: I forgot about Bootstrap! That projects saved the world from millions of ugly web apps and dashboards built by clueless backend engineers.

On the other hand, I kinda loathe Bootstrap. I started web dev a bit after it peaked so maybe that's why but I'm so sick of the look and find it hard to think people ever thought it was good.

Another trend Twitter popularized that I can't wait to die: absurdly large border-radiuses

The world before was still riding high on the euphoria of AJAX and Web 2.0, doing all kinds of whacky things that looked pretty but were completely unusable. Chrome was out there optimizing JavaScript processing so people could do more on the same hardware, and do more they did! It was a nightmare.

Bootstrap helped establish a baseline of usability both for end users and web developers. People who might have been tempted down the path of the latest unusable shiny were kept in line.

But they haven't open sourced their recommendation engine in the past

This release seems less immediately valuable than their other contributions, but historically more significant. It’s a pity we don’t have commits although that would be a huge privacy issue.

But yeah, while I would never work for Elon I’m glad he did this.

Twitter had 18 years to publish their algorithm under the previous management and they didn’t.

Twitter didn't get the algorithmic timeline until 2016. It also wasn't founded until 2006, so I don't know where your 18 years came from.

Well, they didn’t