It can still be achieved, since 2018 I have been playing and I have realized that even new players receive exclusive weapon packs or like the timmy helmet and I do not understand well how it works
You’re able to get almost everything still.
The HCS skins are available for purchase, as are other armour packs.
Most skins are in REQ Packs to be honest, just unlock them over time.
Some Skins are gifted to you by 343i for certain things.
The only Skins I know of that aren’t available anymore are the StickerShock & Black/Yellow 343 Skins.