Let's be honest about what actually went out of fashion. It's the lazy version: three walls in builder's-finish white and one in a clashing feature colour, usually a chimney breast, with no thought behind it. That look peaked in the early 2000s and it does read as dated now.
But accenting a wall with colour? Far from dead. What's changed is the approach. Instead of one wall fighting against three pale ones, the smart move is colour drenching — painting walls, trim, and sometimes the ceiling in the same shade — or using a richer, more considered palette across the whole room so the "accent" feels intentional rather than orphaned.
If you genuinely want one wall to do more work, make it earn it. Pick a wall with architectural logic — a chimney breast, an alcove, the wall behind a bed — and choose a colour that relates to the rest of the scheme rather than shouting over it. Farrow & Ball's Hague Blue or Inchyra Blue behind a bed, with the remaining walls in a soft companion like Cornforth White, still looks cracking because the two tones are talking to each other. Little Greene's deep greens like the Brunswick family, or Mylands' richer tones, work the same way.
The "but what about a small room" question: in a poky space, drenching the whole room in one mid-to-deep colour usually beats an accent wall, because a single bright wall just makes the box-shape obvious. Colour all four walls and the corners disappear.
My honest advice — don't do an accent wall as an afterthought or because you're scared of committing colour to the whole room. That fear is what makes it look dated. Either commit properly to one wall with a colour that belongs, or drench the lot. The half-hearted middle ground is the only version that's genuinely out of style.