@johnmu Mastodon has a search engine opt out setting for each profile which sets a noindex meta tag (https://vis.social/@Luca/109620741021800649). Honoring that and the robots.txt should be enough in my opinion. But as you have seen, many people see that differently.
The majority of people I reach are fine with hashtags in public posts being indexed: https://vis.social/@Luca/109619582185050938