How can I enable backspace to go back in Safari on Mojave?

You can add a keyboard shortcut from the terminal, but this requires you to disable system integrity in macOS 10.14 Mojave and later.

defaults write com.apple.Safari NSUserKeyEquivalents -dict-add Back "\U232b"

Alternative, you can install a Safari Extension which injects a script that bring back the functionality:

https://github.com/yene/Safari-Backspace

For example:

document.addEventListener("DOMContentLoaded", function(event) {
    function handleBackspace(e) {
        if (e.keyCode === 8 && !e.ctrlKey && !e.shiftKey
            && e.target.tagName !== 'INPUT' && e.target.tagName !== 'TEXTAREA'
            && e.target.contentEditable !== 'true' // TinyMCE
            ) {
            e.preventDefault();
            window.history.go(-1);
        }
    }
    window.addEventListener('keydown', handleBackspace, false);
});