Shouldn't that be "Plans B"? I believe "plan b" is a head first compound noun, and should pluralize like "hangers on", "mothers in law", or "attorneys general"

Shouldn't that be "Plans B"? I believe "plan b" is a head first compound noun, and should pluralize like "hangers on", "mothers in law", or "attorneys general". But I could be wrong. The plural form doesn't get much usage.