Traditionally I've labelled my OSS projects 1.0 when they've stabilized and the rate of change has greatly reduced. Right now Mill is not there yet, but maybe if at end-2025 we realize no breaking changes have been needed since end-2024, we can call it 1.0
Just a tiny notice, the github page, nor the website seem to currently contain an “installation” link. The one found by google returns a ‘page not found’ for the current version.