secret_id_key is an RSA key and ed25519_master_id_key is a Ed25519 key used for Hidden Services v3.
Here is what the spec says about it: https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt#n127
0.1. Improvements over previous versions.
Here is a list of improvements of this proposal over the legacy hidden
Better crypto (replaced SHA1/...
As per changelog, Tor, starting with 0.4.6, no longer publishes any DirPort:
Removed features (relay):
Because DirPorts are only used on authorities, relays no longer advertise them. Similarly, self-testing for DirPorts has been
disabled, since an unreachable DirPort is no reason for a relay not to
advertise itself. (Configuring a DirPort will still work, ...
The spec for the v3 onion services, starting with section 2.2, explains it rather well:
2.2. Locating, uploading, and downloading hidden service descriptors
To avoid attacks where a hidden service's descriptor is easily
targeted for censorship, we store them at different directories over
time, and use shared random values to prevent those ...