The repository is not about these kinds of services. It's about websites that send you email notifications or do email support using encryption/signing. For example, if ramble has a public key, they can sign every email they send you (notifications, password resets, etc.) so you can verify you are not getting phished and the email comes from them. Or if you have sensitive info to send them, you can encrypt it before sending it, regardless if you use fastmail, posteo, tutanota, protonmail, gmail or any other email service.

This also doesn't have to be limited to email communications/notifications. If a website decides to only support notifications through XMPP or any other method, it can still apply, it's just that email is the most widely adopted.