The Safari platform does not support the WebExtensions framework and thus is no longer supported. Consequently the code base has been removed.
Note that the code base here was before the official fork Safari fork was created, so it does not correspond to the version of uBlock Origin which could be installed on Safari.
The last commit which contains the code is 917f3620e0c08b722bbd4d400bca2735d9f6975f.
You can browse the last state of the removed code base at https://github.com/gorhill/uBlock/tree/917f3620e0c08b722bbd4d400bca2735d9f6975f/platform/safari.