They grandfathered everyone a couple months before the final cutoff. It was available free during that time — you just had to sign up. I’m afraid you will have to sign up for a pro account
Email from Meetup 6/2019 – “Move to OAuth soon for continued free API access: Until August 15, members will be able to apply for OAuth access free of charge. After August 15, anyone who wants to apply for API access through OAuth will need to have a Meetup Pro account in order to do so.”
From 7/2019 – Apply for an OAuth consumer at no cost (before August 15): After August 15, only members with an active Meetup Pro subscription will be able to create new OAuth consumers.
If I recall correctly, the Nuance plugin was updated during the free period and prompted users to upgrade?