Prudes is a plural of word prude. Because prude person seems prude only among less prude persons there is no situation in real world were anyone needs plural word of prude.
That's why word prudes is only used in about imaginary sentences.
None knows what those sentences actually stands for, but they are used when own behaviour or appearance is imagined less proper and someone elses is correct.
Can be also seen as necessary tool in social behaviour as imaginary numbers are in mathematics.
Not to be confused to phrase "you know what they say about prunes" what actually is just smart advice to use known medicine for symptoms of constipation.
P1. I look so fat in swimsuit.
P2. Same, that's why God invented bikinis for us.
P3. Look that swimsuit Nancy is wearing!
P2. That Nancy is such prude bitch!
P1. You know what they say about prudes.
P2. Yeah.
P3. So true.
M1. That chic from West Coast just cockblocked me in front of her friend!
M2. Man! That's so wrong. Why those west coast chics are always such prudes?
M1. Well you know what they say about prudes.
M2. Yep.