In macOS Mojave, is there a list of unused keyboard combinations for shortcuts?
Is there any table with a list of unused shortcuts in Mojave that could be good candidates for custom shortcuts? 


There is not. There are a few utilities out there which will show you which key combinations are in use¹ but none (that I am aware of) show you available keyboard shortcuts.

¹ I believe KeyCue is the most well-known and best supported.

One common method among Mac power users is the use of a Hyper Key which is (generally speaking) where you remap holding down the Caps Lock key to equal holding down ⌘ ⇧ ⌥ ⌃ because few apps (and few macOS default shortcuts) use all 4 modifiers (mostly because it is awkward to try to press them all at once, which is why remapping the Caps Lock key is recommended).

Personally, I use a Hyper Key with Keyboard Maestro because, unlike shortcuts defined in System Preferences, Keyboard Maestro's shortcuts are easy to backup/restore, and easy to sync across multiple Macs.