I seem to have Composer 2.0 ... but I can't see that it's been released yet?

2.0-dev+df0cada93f1c1f14e2503f1d943824cf27535acb

I reinstalled an old VM yesterday afternoon and updated everything as normal. Then when I wanted to pull in packages for my legacy project I got a bunch of PSR-4 errors:

Class does not comply with psr-4 autoloading standard. Skipping.

I understand that this is a breaking (but necessary) change in v2 ... but I didn't expect to just "get" v2 without any warning? According to the downloads page (https://getcomposer.org/download/), the latest version is v1.10.5