Just as a warning, this is licensed under the AGPL, with a CLA that requires copyright assignment. So, they could pull the rug at any time:

2.3 Outbound License. Based on the grant of rights in Sections 2.1 and 2.2, if We include Your Contribution in a Material, We may license the Contribution under any license, including copyleft, permissive, commercial, or proprietary licenses.

Monal comes close.

nlnet is the main one that comes to mind.

Any new open source software is always a net positive.

But, there are a few small caveats to the way they’ve done it (depending on how cynical/cautious you are):

• Because Proton are not accepting contributions, they own all the copyright, so can make the code closed source again if they want to (that wouldn’t affect the already released versions, but future versions)
• They could likely take down any derivative on iOS, since Apple will always take instruction from the copyright holder, for GPL’d code
• Since the builds are not reproducible, there’s no guarantee that the binaries they distribute are built from the source code