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@cliffwade The most immediate change we are making is disabling registrations on newly installed instances, and displaying a banner to state that open registrations require a reactive moderation team.
Captcha support has been introduced in 4.2 (in reaction to another abuse wave), but we need to improve it. It was initially intended to be automatically enabled when an abnormal rate of signups was detected, but we did not had time to finish this (see https://github.com/mastodon/mastodon/pull/25032)