Steel (chromoly) frames are absolutely still relevant for mountain biking, and they offer some unique advantages. The biggest selling point is ride quality—steel has a natural springiness that absorbs trail chatter and vibrations better than aluminum, giving you a smoother, more comfortable ride over long distances. Steel is also incredibly durable and, unlike carbon fiber, can be repaired by a skilled welder if you ever crack or dent it.
The trade-off is weight. Steel is the heaviest common frame material, which matters more if you're racing or climbing steep terrain. It's also prone to rust if the paint gets chipped and the frame isn't treated internally, though modern steel frames often have anti-corrosion coatings inside.
Steel makes the most sense for hardtail trail bikes, bikepacking rigs, and dirt jumpers where durability and ride comfort outweigh the weight penalty. If you want a bike that lasts decades and feels great on long rides, steel is a solid choice.