They can. For Mastodon there's an option for "Opt-out of search engine indexing" which adds a "noindex" tag to your profile which Google respects. Not all indexers respect the tag however and "noindex" data isn't federated which can cause your replies to show up: https://github.com/mastodon/mastodon/issues/22047
Replies to posts can be indexed by search engines even if I've opted out of search engine indexing · Issue #22047 · mastodon/mastodon
Steps to reproduce the problem Check the "opt-out of search engine indexing" option (Preferences/Other) Make a public or unlisted reply to a public post by somebody who doesn't have that option che...