Based on this closed issue, I don’t believe Lemmy natively keeps an edit history, the comment is overwritten. However, admins have full access to all data on the server database so they can (and do) keep backups and look over them at any time. You should consider everything you do on your instance available to your admin, including private messages.

If it’s been online for any amount of time, always assume that someone has seen it, screenshotted it and asked the wayback machine to archive it.

Chances are they haven’t, but if you’ve leaked a password or key, better change that right away.

Everything you send to the server

Everything.

Don’t forget it may have also been replicated to other instances before you edit or delete it where those admins also have access to the database.

Well if it’s a post, that’s not private information anyway, even if you edited it. As for actual private data, they can see your email address.

Can they see passwords? If so, I recommend not using the same stuff for every instance.