mastodon.social is one of the many independent Mastodon servers you can use to participate in the fediverse.
The original server operated by the Mastodon gGmbH non-profit

Administered by:

Server stats:

311K
active users

Confirmed. ChatGPT is actively indexing the Fediverse, even small servers like mine who have not explicitly consented to their indexing.

So while people on Mastodon got angry about Mastodon having built-in discovery features, ChatGPT just went ahead and slurped up all your posts.
Nathan A. Stine

@atomicpoet if people don't want this, the only way to guard against it (IIRC) is for their instance admin to enable AUTHORIZED_FETCH.