Team Fortress 2 is also listed as Free-to-play.
Yet a VAST majority of game content (i.e hat & trading) is only available after a single purchase.
Also, going by the Valve's definition of free to play, you don't HAVE to buy the expansion missions to continue to play the game, therefore, Spiral Knights can and is still considered Free-to-Play

SK is still free-to-play. It just can't brag that "you can access everything F2P" anymore. Basically, with this update it has started down the road of pay-to-advance-past-level-20. If OOO can get away with this update, you can be assured that they will put out more pay-only content in the future, and that makes me sad. If pay-only content includes powerful, balance-changing equipment (still too early to judge the Crimson Hammer items at this point), then it will go down the drain even faster. It'll take a few weeks to figure out the ripples from this update affect things, but I guess we'll see how this plays out.