I loved earning armor in games like Halo 3 and I’m happy to hear that’s somewhat still a thing. All I know is the ODST helmet came from nightfall and I’ve heard about this whale one and the other one (I forget the name) you earn through spartan company commendations.
Pretty much everything is unlockable except for these few helmets and armor.
Achilles( which can be attained with lots of time and effort with a full Spartan company)
Timmy the whale helmet( which can be attained by some new method that just got released there’s several people of got it but right now the theory is that you have to play lots of custom games and some Forge I think too)
Olive pig Helmet ( you can only get this helmet if you get one of your Forge Maps into matchmaking)
There is also two sets of HCS armor that can only be obtained by buying a HCS pack which are not available.
And also one last thing the helmet that you receive from the Nightfall miniseries is actually a Nightfall Armor Set.
Anything that isn’t HCS branded (Athlon Champion and Challenger) can be unlocked through normal REQ packs, with the exception of Achilles. Helioskrill and Nightfall used to be only available through special means - the former requiring you to beat MCC on Legendary, and the latter requiring you to buy and watch all of Halo: Nightfall - they can be bought through normal Gold packs.
The Olive and Timmy helmets are usually reserved for prominent Forgers as merit for their work, though the latter is being given out under undisclosed conditions.
Achilles (the armor set earned through Spartan Company commendations) remains the only set that is strictly earned. As long as you can handle the grind, you are “guaranteed” to get it.