OpenSpec maintainer here. A lot of our adoption has mainly been bottoms-up, it's usually driven by engineers. That being said it definitely helps if everyone on the team uses it together. Especially when shifting left and doing a lot more "spec review".
Thanks for taking time to respond here. Would love to know from your experience the scale of function-points, team size, client-requirement variance, etc. different teams would have worked with and maintained over a period of time via this open-spec.
Please note that I can already see that github repo has 68k+ stars. So popularity is not in question, just the viability and consistency of adoption across different scenarios.