Andrea Veri commented:
@tomtryf, disabling captcha is unfortunately not possible on a per-list basis, I wasn't aware of the SUBSCRIBE_FORM_SECRET config variable tbh, that's something we surely want to look into to eventually disable captchas on the whole mailman instance and drop the external dependency on proprietary software. For now I'm going to re-enable the captcha as otherwise new user subscriptions will fail, it's on my to do list to look into this further. Thanks!