But what are the upsides of this change?

The problem with httpx as a dependency is that it's currently working towards a 1.0 release which will be full of breaking changes.

The httpx2 project is essentially a fork that promises not to break the existing API, which makes it a more stable dependency to build against.

I wrote a pretty long comment about my concerns for the breaking 1.0 version last year - https://github.com/encode/httpx/discussions/3344#discussionc... - in that comment I recommended the HTTPX project release their 1.0 as a package called httpx2 instead, but a year later we now have an httpx2 released by a different maintainer) that keeps the old API instead.