Steelix is great in GSC because it tanks both Snorlax and Electrics all in one (at least their most common sets). Since Snorlax, Zapdos, and Raikou are the three best mons in the game, a Pokemon that can switch into and match up well against them all is going to be great. Steelix in GSC runs a basic Earthquake/Curse/Roar set, followed by one of Rock Slide, Body Slam, or Explosion, each of which has its merits. It's fantastic with Spikes down and a true terror in the endgame, although when trying to open things up, Steelix is pretty mediocre. Also worth noting is that it's arguably the best emergency Phazer in the game, as it can switch into and Phaze both AgiPass Umbreon and Curselax, in addition to Thunder Missy and ML Pass Umbreon, and thanks to its status-immune typing, it can do it all without being whittled away by Toxic or screwed over by a full paralysis. As far as ADV, it declined in usage as everyone flocked to Skarmory/Metagross as their Steel, or Swampert/Claydol for their Ground. Steelix's main turn-off relative to other defensive Pokemon is that it's weak to Earthquake, and of course its offenses are too weak for it to try to find a niche in that kind of role. Steelix is still good, though, and the toughest Rock Slide absorber you'll ever find, and all-in-all is a decent Spikes shuffler (much like its GSC counterpart). Steelix in DPP is a decent Phazer/Rocks setter in lower tiers I guess, I'm not entirely sure where it fits into DPP. The main thing keeping it out of OU, though, is that the metagame shifted from one where power and defenses were king to one where speed was king. Steelix is REALLY slow, so naturally, it's pretty limited.