Since we're talking about reducing unit classes, here are a couple more possibilities I noticed.
Missile and Doomsday: There is not much difference between these two, and the things you might expect to matter actually don't matter. Units as cargo is a function of the Special Unit type, not the Unit Combat type. Promotions also do not matter, as the Doomsday units all have a combat strength of 0 and therefore cannot get any promotions. The only difference that I could find is the Python code that is used for the Nuclear Protest event and the Nanite Defuser wonder. Both of these can destroy Doomsday units. We can identify nuke units with unit.nukeRange() > 0 and destroy them that way. We really don't need to check unit type.
Naval Units: We have five classes of naval units: Wooden, Steam, Diesel, Nuclear, and Submarines. I am not sure that the middle three add any distinction.
I want to keep the Wooden Ships distinction. The main two reasons for this are Asatru and the Maneuvering promotions. Ocean tiles have a movement cost of 2. Maneuvering I reduces this by 1, so wooden ships without Maneuvering move slowly over the ocean. It's also very difficult to get to 5 XP for Naval units before the mid-Renaissance, when Magellan's Voyage and especially Naval Academy come available. I think Feudal + Intolerant, or a settled Great General, is about the only way to get it. So while Caravels can explore the ocean starting in the late Medieval, they will not be quick about it.
All of the Industrial and later ships ignore terrain movement costs, so they don't need Maneuvering at all.
Asatru Monastery specifically gives XP only for Wooden Ships. I designed it that way to be similar to Monasteries that start to obsolete at Humanism before going totally obsolete when replaced by School or at Computers. Asatru Monastery loses part of its ability when Steamships replace Wooden Ships and the rest later on.
Submarine also has a use as a class, to let Destroyers get a bonus against the class without having to write out every single unit and to make Sonar work as a promotion.
But I'm not sure about the middle 3 classes; they only seem to benefit from the bonuses that later ships get against earlier ones.