Salesforce supports an OAuth flow called "Client Credentials Flow" described here. It is specifically designed for Server-to-server integrations.

The requirements on Fivetran's side is simply to capture the client_id and client_secret values provided by the Salesforce "Connected App".

Note that these fields are already required for the new "Advanced Authentication" feature. So the only difference is that mTLS is not required. Essentially, this feature request is simply: make mTLS an optional part of the "Advanced Authentication" feature; simplifying the integration (enabling mTLS within Salesforce is not trivial).