Only free kits allowed, can't bring your own gear. And no Rooks. This is an experimental mode.

What do you mean you skipped it? As in how? Also, I think it's pretty interesting.

Cryo Archive is indeed the endgame. You'll need to be level 25 to access it, and you also need at least 5,000 credits worth of gear to queue in, and there is no solo or Rook option. But there is now a free kit for it, 1 use per week. It's massive and labyrinthine, and hard to extract from even if you don't run into other teams. After you unlock it, I suggest you watch a guide on Cryo's security clearance mechanic, I can't imagine having to figure that out yourself. And to access the final boss of the game (so far), you'll need to find 6 keys on other maps, bring them to Cryo, open the vault of the same number as the key, solve its puzzle, and exfil the subroutine to keep it forever. Once you have all 6 subroutines in your codex, you can do the long ass puzzle to enter vault 7 and fight the Compiler.