Halflings are not locked out of anything. The best Paladin and the best Ranger I have played were both small. The only thing they are really "locked out" of is a grappler build, and that is not really a class.Huh. I wonder why a species that is more or less locked out from like three classes due to garbage size rules might not be played as often...
If you are playing RAW with Tasha's moving ability bonuses they are competitive with any other PHB race for really any class. If you are playing with critical fail homebrew they are hands down the best race for any martial, including all the official races.