Long lived feature flags are totally fine, they're more like operational flags than anything. The Fowler article is pretty good at classifying them. Depending on the type of flag (longevity/dynamism) the design will vary. https://martinfowler.com/articles/feature-toggles.html
An essential property of a feature flag is that it is short-lived, existing only for the duration of the roll-out of the feature. In the language of your linked article, feature flags are 1-to-1 with "release toggles" and not really any other kind of toggle.