I’m sorry if you don’t like it, but it meets all the technical requirements for being a proverb. It’s considered a proverb by Charles Doyle, Wolfgang Mied, and Charles Shapiro* in their Dictionary of Modern Proverbs; their earliest printed citation is 1977. So, not terribly old, but s old enough to be called “traditional.”
*It’s an obscure field, but in it these are names to conjure with. In other words, not just a random publisher-assigned editorial team here.
Well, I’m not impressed by a standard that calls something “traditional” when its first printed citation is well within the living memory. I would think that it would have to survive at least one completely dead generation in order to be traditional. Preferably two.
So you’re saying that your feelings are more reliable than scholarly consensus in the field devoted to the study of tradition? Great. Way to fight ignorance, dude.
How would that even work? If it’s not a proverb, what is it? How would new proverbs ever arise if they could only become traditional under your conditions?
A. Large toilets don’t necessarily correlate with massive stool. Your argument thus still requires more evidence.
B. A Norwegian proverb says it’s the dumbest farmers who get the biggest potatoes. Now this is actually about actual potatoes, but I’m willing to stretch the interpretation and pretend it’s a well known euphemism, just to win this battle of “wits”. 
Fighting ignorance doesn’t mean accepting on faith every standard set forth by scholars, especially when such standards are necessarily somewhat arbitrary. And language is not defined by scholars, in any case. It’s defined by use. And if enough people come to agree with my “feelings” about the use of the word “proverb,” then, yes, my feelings are more reliable than scholarly consensus.
What is “Close the door on your way out” or “Turn the lights off when you leave a room” or “Wipe your shoes before coming inside” or “Wash your hands before dinner”?
New traditions would arise by surviving at least two dead generations. That’s not so complicated.
I’m not asking you to take anything on faith. Certainly don’t take my word for it: I’m just some schlub on the internet. But look into it. The scholarship on proverbs has reasons for what they accept as a proverb and what they do not, and your ignorance is NOT equal to their expertise. When you have some expertise, feel free to disagree, and by all means be skeptical while you’re gaining it. Just don’t reject it out of hand because it sounds wrong or disagrees with your preconceived notions.
yeah, the “proverb” may date back to 1977 - but I’ve never lived in an area that had water shortage issues - except a few summers in some areas around Toronto, I think. As a result, I’ve never seen issues about “don’t flush unless you have to”. The first time I ever heard of that saying was in Meet the Fokkers. I imagined it was for places like Arizona or the Florida Keys whre water shortages are a real issue.
In fact, just the opposite. I’ve been trying to train myself to turn off the tap while brushing my teeth, but the way I learned was to leave the water flowing (usually pretty fast) during the whole process of brushing my teeth; ditto for shaving. I never understood how rinsing a razor in a sink of water full bits of whisker actually got it clean. I left the water running the whole time I was shaving.
Plus, I ahve an extra-large, deep bathtub.
When I was in school, it was not uncommon for public washroom urinals to flush ever automatically every 15 minutes or so. Mostly that’s been replaced by sensor-flushers now.
Obviously, my behaviour is not good behaviour in a land of water shortages; but then, some countries some areas there is no shortage of relatively clean fresh water. They do meter and charge for it, but a few dollars more each month is somewhat irrelevant.
I don’t really have to as our toilets generally are quite efficient. As it happens, we don’t even have that word in Swedish.
Which word? Flush?
Flooshie-bork-bork?