I've never seen anything longer in 3DS but IIRC, the file specs don't state it explicitly and jPCT's loader would load longer names too...there just aren't any files using them. Maybe it's good practice to keep them 8.3, because anything else would break a lot of loaders that rely on the length.
This limitation always bites me when doing the quake3->3ds conversion, because without anything else, you'll end up with a 3ds that links to textures named "TEXTURE_.JPG" and similar, which is useless...