You could argue the opposite, that Twitter is part of the open internet to some extent: unlike Facebook, it is fairly searchable with normal search engines [1] and unlike Instagram, it doesn't actively discourage linking to other websites. Long time ago, it even used to offer RSS feeds [2]. There are custom clients for it too. Apparently, Jack tried to push for decentralized social media protocols [3]. However, I would certainly agree that it is getting closer to a being yet another brick in the wall.

As for free expression on the open internet, I think it is under active threat. The centralization [4] makes the job of the censors much easier. The calls for internet surveillance and censorship are ubiquitous.

[1]: e.g. https://google.com/search?q=site%3Atwitter.com+jack+open+int...

[2]: https://archive.md/IPLAx#selection-215.17-215.123

[3]: https://cointelegraph.com/news/what-does-twitters-new-decent...

[4]: e.g. https://easydns.com/blog/2020/07/20/turns-out-half-the-inter...