A Supporters page can have one free tier. Anyone with a Magpie account joins it from the public page at once; nothing is charged and no Stripe account is needed, so a page with a free tier alone can be published.
Posts have a new audience, All supporters, which includes the free tier. Paying supporters and Chosen tiers keep their meaning. A free supporter who joins a paid tier is moved onto it, and a free supporter can leave from Who you support.