There is an issue, give a thumbs up here:
https://github.com/mastodon/mastodon/issues/10590
(This is an excellent issue by the way, it suggests some good ideas to help new users alongside the request for making approval default.)
To be fair to the current version of Mastodon, it does now warn admins not to use open signups unless they have a strong moderation team, but it seems some admins ignore that warning. 😦
@homegrown thanks for sharing, my upvote is there, hope more admins check it too! cc @spla @jerry