Full disclosure, I am the CTO of Flagsmith, and we have seen a clear curve in adoption of OpenFeature over the last few years. It used to be that we were pushing customers to try it out, now they come to us with OpenFeature as a requirement.

The vendor support is pretty mature now and there is coverage across almost all languages. If you're integrating feature flags into a new service, or looking to migrate from e.g. home-grown to a third party solution, OpenFeature is definitely the way I would recommend going.