How to make an ethical Mastodon/Fediverse search engine:
1. Run a bot that follows back everyone who follows it
2. Index the public toots that bot receives from those that it follows and nothing else (i.e. no followers-only or unlisted toots)
3. Honor all edits and deletes, and remove toots from folks who unfollow the bot
4. Do all the above with full transparency
Alternate, easier approach:
1. Don't