Zone 2 is the current name for brisk, conversational, moderate-intensity cardio done steadily. Whatever the label, the underlying behaviour is the one every guideline measures: regular moderate aerobic activity. On that behaviour the evidence is about as strong as lifestyle evidence gets.
The WHO 2020 guidelines on physical activity grade the link between regular moderate-intensity aerobic activity and lower all-cause mortality, cardiovascular disease, type 2 diabetes and depression at the highest confidence level, drawing on huge cohorts and consistent dose-response data. The branding is new; the recommendation is not. No long study has isolated 'Zone 2' as such, so the grade attaches to the activity, not the hashtag.
Doing it: The guideline floor is 150–300 minutes a week of moderate activity, which is three to five sessions of 45 minutes at a pace where you can talk but not sing. Cycling, brisk walking, easy running and rowing all qualify. More is better up to a point; some is vastly better than none.
Who it suits: Everyone able to move. Start where you are; even 10-minute bouts count towards the total.
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