There was a Captain Jack song in a DDR game as recently as Dance Dance Revolution SuperNOVA CS. None in SuperNOVA 2 or X, and don't ask me about X2 or any of the home releases on systems other than PS2.
The arcade mixes have worked the way you're describing for some time now, letting you access almost any song from a previous arcade mix (meaning nothing from specialized mixes and none of the US CS-exclusive tracks, among other things).
Fun fact: DDRMAX has at least two songs ("Follow Me" and "Flash In The Night") that have never been in another DDR game.
Also: "I'm a cowgirl, and I'm ridin' on my horse."
And: I'd say it's not anywhere near 90%, if only because a lot more than 10% of the songs in the games are instrumentals (to say nothing of how many songs have "PARANOiA," "Trip Machine," or "MAX" in the title).
(And I'm not a fan of StepMania nowadays. No reason to play official songs on there if you can't use a pad, and keyboard simfiles are retarded, because it's basically four-button beatmania at that point, except with anime music.)