I have been intentionally very conservative with what goes into a patch release. The reason is that with the standard setup the latest patch version gets used automatically on CI, and so it is easy to silently and automatically break the deployed documentation. E.g. in case of 0.27, e.g. the KaTeX library had a major version bump, which might break the rendering of some more complex equations.
As for 1.0 – yes, that would be good. It would allow for a bit more nuance in terms of versioning (more complex docs probably want to restrict to a minor version of Documenter, but simpler ones are probably fine with the standard a major version restriction). But there is a backlog of things that should be done before we commit to 1.0 (e.g. moving the Markdown backend out of Documenter, revising the documentation).