There are various parts to Theseus' armour of course, and they do seem to suggest you need it to defeat the Minotaur, but it's only blue armour with no bonus perks for a full set, so don't worry about it too much.

Does Theseus armor do anything?

Now that the Theseus Armor has been collected, Players can go and take on the Legendary Beast Minotaur. The Theseus Armor will provide added protection from the attacks by the Minotaur and make the taxing battle against this beast a little easier.

What level should you fight the Minotaur?

The Minotaur is a level 40 enemy, so don't even try to kill him until you're around level 37 at least. The final mythical beast is the hardest in Assassin's Creed Odyssey. To get this mission started, you'll need to find the "Romancing the Stone Garden" mission in Lesbos.