For example, nobody likes to use the restroom after they take a shower because you slide around on the seat, but that's not something commonly talked about between persons.
Or the well understood, but un-discussed feeling you get when you feel a bug or something crawling on you before you fall asleep, even though you know it's not a bug, but just a sensation.
It could not be of English origin.
Open secret — M-W
noun a supposedly secret but generally known matter
"It's an open secret that he's been cheating on his wife."
Open secret — Wikipedia
An open secret is a concept or idea that is "officially" secret or restricted in knowledge but is actually widely known; or it refers to something that is widely known to be true but which none of the people most intimately concerned is willing to categorically acknowledge in public.
It's not single-word, but there's a good fit in common knowledge.
something that is known to many people but often not made known officially
As you say "It could not be of English origin", the adjective taboo comes to mind.
late 18th century: from Tongan tabu ‘set apart, forbidden’; introduced into English by Captain Cook.
Definition: "not acceptable to talk about or do" (Mirriam Webster)
examples:
Filipina helper Liz (not her real name) battled bed bugs in June 2012 in Tai Wong Street in Wan Chai. She says the pests covered the 500 sq ft hostel, which was home to 10 helpers. "They were on the walls, on my towels and on the carpet." Travellers also spread the problem. Bed bugs are a bigger issue in hotels than the industry will admit. "People don't talk about it because it's taboo, but it's definitely happening" http://www.scmp.com/lifestyle/health/article/1402401/bed-bug-infestations-are-rise-and-pests-are-hard-kill
Get kids talking about the loo, say NGOs, and it will help break down a major barrier to education in the developing world: cultural taboos about sanitation. https://www.theguardian.com/global-development-professionals-network/2015/nov/19/reading-writing-and-sanitation-how-kids-are-key-to-ending-toilet-taboos
unspoken truth (thanks be to @hatchet)
Also, as a variation on the term unspoken rule, we could coin the phrase unspoken culture.
Finally, Carl Jung's collective unconscious seems apt here.
According to Jung, the human collective unconscious is populated by instincts and by archetypes: universal symbols such as the Great Mother, the Wise Old Man, the Shadow, the Tower, Water, the Tree of Life, and many more.
Jung considered the collective unconscious to underpin and surround the unconscious mind, distinguishing it from the personal unconscious of Freudian psychoanalysis. He argued that the collective unconscious had profound influence on the lives of individuals, who lived out its symbols and clothed them in meaning through their experiences. The psychotherapeutic practice of analytical psychology revolves around examining the patient's relationship to the collective unconscious.
Tacit \Tac"it\, a. [L. tacitus, p. p. of tacere to be silent, to pass over in silence; akin to Goth. [thorn]ahan to be silent, Icel. [thorn]egja, OHG. dag[=e]n: cf. F. tacite. Cf. {Reticent}.] Done or made in silence; implied, but not expressed; silent; as, tacit consent is consent by silence, or by not interposing an objection. -- {Tac"it*ly}, adv. [1913 Webster]
The tacit and secret theft of abusing our brother in civil contracts. --Jer. Taylor. [1913 Webster]
Perhaps narrower than you are looking for, but unmentionable comes to mind. Unmentionable typically connotes the subject is well known, but not spoken about, due to embarrassing or offensive aspects.
I say "perhaps narrower" because your question doesn't limit the lack of discussion as being due to an embarrassing or offensive characteristic.
I'm at a loss for a single word, but this type of thing is commonly described as "something people don't talk about." "Something people don't talk about" isn't necessarily taboo; it's just considered unpleasant, uncomfortable, or rude to discuss with others, though there is an unspoken, implicit understanding that most people have knowledge of or experience with the unmentioned "something."
As a single word, I would refer to the "something people don't talk about" as an unpleasantness, though by definition it isn't necessarily "something people don't talk about."
Example:
The unpleasantness of accidentally getting poop on your hand when changing a baby's diaper is something people don't (usually) talk about.
Your examples seem different to me.
For example, nobody likes to use the restroom after they take a shower because you slide around on the seat, but that's not something commonly talked about between persons.
I would say that's probably considered impolite to discuss, like the details of your bowel movements. You probably wouldn't discuss it at a business meeting or the dinner table, but you might with your doctor. ("I sprained my wrist when I slipped off the toilet...")
Or the well understood, but un-discussed feeling you get when you feel a bug or something crawling on you before you fall asleep, even though you know it's not a bug, but just a sensation.
I don't know of any social barrier to talking about that; it just might not be interesting enough.
The reasons why people shy away from discussing such topics vary, so corralling all of them into a single term is challenging. grateful mentioned taboo in another answer. Other broad terms for subjects that may make others uneasy are rude or gauche.
“Toilet humor” is not discussed in polite company because it is considered tacky. Sex jokes and objectifying humor are off-color, crude, or obnoxious. We don’t generally announce the results of colonoscopies to or share unedited, graphic birth videos with mere acquaintances because the topics are uncomfortable or awkward. Revealing one’s exact income to someone whom the speaker has not engaged in an accounting or legal capacity is seen as boastful, unfiltered, or—itself crudely characterized—douchey.
The last is interesting because it is borrowed from an unmentionable (also mentioned in another answer) bathroom subject as a pejorative for immoderate behavior.
