My understanding when I looked into this a couple years ago is the optional wider scope search is only possible to enable for one's own posts/account and while logged in[1], but still limited. At a server level the only search possible (using the regular Mastodon) is for user-added tags on posts rather than full text and this was an intentional decision[2].

I'm not familiar with indexability outside Mastodon instances, though can't really recall coming across them naturally in search.

[1] https://docs.joinmastodon.org/admin/elasticsearch/

[2] https://www.reddit.com/r/Mastodon/comments/yv1cl4/global_sea...

Edit: updated with links from my prior research.

Does this mean indexing within the database that drives this Mastodon instance, or indexing by web crawlers such as googlebot?