It is not a subculture

From Peter Durfee via Twitter:

No, you're right, it certainly isn't that. pic.twitter.com/HYGCQ6whvx

— Peter Durfee (@Durf) February 19, 2016

Oh, my, this is one of the nastiest translation failures I have ever encountered.

Sonaetsuke no toiretto pēpā igai wa, mizumore/tsumaru gen'in to narimasunode nagasanaide kudasai.

備え付けのトイレットペーパー以外は、 水漏れ・詰まる原因となりますので流さ ないで下さい。

Because [anything] other than the supplied toilet paper causes leakage/blockage, please do not flush [anything other than the supplied toilet paper].

Alternatively, "Please don't flush anything other than equippd toilet paper since it may cause water leakage or clogging."

Or, more colloquially: "Don't flush anything other than the toilet paper, as it may cause leakage, etc."

Nathan Hopson further explains:

I think the best version of this I've seen in English is: "Please flush only toilet paper and what nature provides"

The "excluding the toilet paper of equipment" is a slight, but intelligible, mistranslation of "the equipped toilet paper" (sonaetsuke no toiretto pēpā). After that, the forces of evil clearly take over…

I agree with Nathan that the last part of the English is a mistranslation of the first part of the Japanese.  The rest of the English (the first five words) amounts to sheer gibberish, perhaps some sort of copy-paste catastrophe from an online translator.

You can see where they got the "equipment".  It comes from the sonaetsuke 備え付け ("equipped; supplied") at the beginning of the notice.

I suspect that "It is not a subculture" may have morphed from something like "uncivilized", "uncultured" they had in mind.  In Chinese we would say "bùwénmíng 不文明", "bùwénhuà 不文化", or something like that.  Not knowing for certain how it would be said in Japanese, I asked Cecilia Segawa Seigle, and she replied:

I don't know how the strange translator got a word like "subculture", whether he meant it to be "uncivilized" or "uncultured." There is no word corresponding to that in the Japanese notice.

Also, just to respond to your explanation, we don't use f ubunmei/myō 不文明 or fubunka 不文化. We understand these words but they are not in our daily vocabulary. For "uncivilized" we use words like "mikai 未開" or in conversational Japanese "okurete iru 遅れている" meaning "he's behind." If he is very behind, he would be called "yaban 野蛮" which is really savage! Okurete iru 遅れている can also be used when a person is intellectually challenged. But you can also say tokei ga okurete iru 時計が遅れている。("the clock is slow / behind").

[Romanizations supplied by VHM; ditto for the translation of the last sentence; forgive any errors.]

I would say that the best part of this English notice is that it functions like a Zen koan and will help you keep your mind free of the mundane reason why you are sitting in that location in the first place.

[h.t. Ben Zimmer; thanks to Hiroko Sherry and Miki Morita]