In June the social dance I help organize decided to drop its requirement that people be vaccinated and boosted:

Checking vaccination cards at the door requires an additional volunteer, is a hassle for attendees, and at this point the communities we draw from are overwhelmingly vaccinated. While we still recommend staying up to date on vaccinations (including your flu shot and covid booster in the fall!), we're dropping this requirement starting with our July 2nd dance.

One question we got, both in person and electronically, was why not keep requiring vaccination on the honor system? Speaking for myself and not BIDA, I think this is rarely a good policy for events.

(I'm setting aside the question of whether vaccination is helpful in reducing risk to others, and writing all of this under the assumption that it is. I do think this is unclear, especially if a policy does not require a recent booster.)

The issue is, an honor system is a bad fit when getting caught is very unlikely and the harms of violating the rule are diffuse and unclear. If an unvaccinated dancer ignores the policy and does happen to bring an infection to the dance that vaccination would have prevented, it's very unlikely that anyone will ever know. There's no feedback mechanism promoting honesty here. Instead, you're filtering unvaccinated people for ones who are ok violating the policy, and nudging more unvaccinated people toward being dishonest in this way.

Compare this to a dance in a small community that does payment on the honor system: an unattended fiddle case by the door for you to drop your money into. If someone doesn't contribute it's much more likely that people will notice, and it's also much clearer that when you defect you're hurting people. There's still some pressure toward dishonesty, but there's enough pushing the other way that it still works well.

The honor system can work well in some cases, but I don't think this is one of them.