From Twitter’s official blog:

Last week, Apple announced Project Catalyst for macOS 10.15 Catalina, which makes it easy for developers to bring their iPad projects to macOS as native Mac apps. We are excited that Project Catalyst will enable us to bring Twitter back to the Mac by leveraging our existing iOS codebase. We’ll also be able to add native Mac features on top of our existing iPad experience, while keeping our maintenance efficient as we continue to improve this shared codebase in the years to come.

As to why Twitter dropped their Mac client in the first place:

Historically, Twitter had a Mac app that shared the same roots as our iPhone app. Over the years, Twitter for Mac and Twitter for iOS diverged as Twitter increasingly focused on its mobile apps. Supporting the two separate codebases was no longer a sustainable option and ultimately we sunset the native Mac app.

And:

The new Twitter for Mac app will use our existing iOS codebase, rather than being built from a separate codebase, following the same successful strategy we’ve used with Web to expand our supported clients. By supporting key Mac-specific behaviors on top of our iOS code, we will be able to maintain feature parity across our iOS and Mac apps with relatively low long term maintenance costs.

Can’t help but wonder if this is an unintended positive consequence of Catalyst, or if bringing apps like Twitter back to the Mac was part of the drive behind Catalyst in the first place.

No matter, glad to have Twitter making its way back to the Mac.