Absolutely! I hung my belt on the doorknob. Also, one of my roommates had the cardboard cutout of Church the cat from Pet Semetary with the light-up eyes. We put in the window, and turned on the cat’s eyes when there was something afoot.
In my fraternity in the 80s, a tie on the doorknob was the standard signal for “seek another place to sleep.”
Yes, 100%. I’ve got 2 daughters in college currently, both have/had roommates in their dorms. The older had an assigned roommate in a double freshman year, then picked roommates for a quad sophomore year, is in an apartment now. Younger is currently a freshman in a double with an assigned roommate also. Nieces & nephews who went to college in the past 10 years or so all had similar experiences. Some schools have singles, but those are almost impossible for 1st years to get.
In three years of having dorm roommates, it was never discussed and no system was made. It also didn’t come up much. By the time it would have been a regular issue for me, I had a single. Never saw any wayward socks or ties on other doors that I can remember but it’s been a long while.
Of course that’s the other difference between the US and UK. The university only typically provides accommodation for the first year, after that you find a house nearby (or that was typical where I went to university in Manchester, a big former industrial city with lots of cheap terraced houses, at least when I went there in the 1990s).
Sure, all of the women I dated in college had systems for alerting their roommates that privacy was expected. More than once, a woman hung something from the doorknob so we could get down to business without anyone barging in; similarly, more than once did we have to find somewhere else to do our thing because the signal was affixed to the door.
Depending on the location, colleges vary on guaranteed housing for anywhere from 1-4 years. My older daughter is in Boston, her college guaranteed 2 years only. Her sister is in a medium sized town in PA, they guarantee all 4 years, though plenty of students do find apartments as they hit upper classes.
We had a ‘Do Not Disturb’ sign that I’d taken from the hotel I stayed at before move in to my freshman dorm.
It wasn’t just for hookups. It was also a hint to not barge in with your friends if I was on a long messy phone call with family or friends back home.
I’d just like to point out that the outcome of a sock on a doorknob is far better than a sock full of doorknobs.
Fair point.
Probably why we always agreed on the hand towel signal.
We would write “BUSY” on the dry erase board.
Back in the 70s, I heard of the sock/tie/towel trope, but never saw it used.
I’m aware that many colleges have changes to the “suite” construct, but I’d be surprised if al schools were able to refit all of their dorms. I can’t imagine how most of the dorms in the Big 10 school I attended could be rebuilt barring HUGE expense.
Most summers I attend a music camp at a small liberal arts college. The dorms are absolutely primitive cells. 2 beds per tiny cement-block cell, w/ tiny shitter/shower between each pair of rooms. Hell, these cells didn’t even have closets - just an open clothes rod in an alcove! 
I don’t think retrofitting could possibly work. The university I went to, some of the newer dorms went the suite concept. I just looked through my school’s housing options, and they were vast majority doubles, with some triples and singles thrown in. There is one large dorm that is all singles, though. When I was there, that was typically taken by juniors and seniors who didn’t want to move off-campus, as the demand for housing there was high, though it did have a somewhat negative reputation for being an anti-social dorm. And there is a smaller dorm/on-campus residence that is all singles.
The newest dorm, which opened a couple years ago, houses over 400 and does have a suite concept, but with two doubles per suite. My dorm recently got completely renovated, and they did not change the layout at all – it’s still mostly doubles, with a handful of singles (usually for RAs) and it looks like they got rid of the triple.
I was in the U. of Evansville in '67 and '68, and opposite-sex guests weren’t allowed in dorms. So, the situation didn’t arise. I later heard about the tie on the doorknob custom. Today, fewer people wear ties, so I guess a sock is the next stage.
Maybe a plastic bag on the knob would mean, “We’re using condoms.”
We had “code words” to be written on the door’s message board if one was “busy”. One guy’s room had a “Do not Disturb” sign lifted from a hotel. So, yeah, this was common when i was in school.
When I was at NMSU in the late 80s. there were several, gender-segregated dorms on campus. Periodically, they would have surprise fire drills. It was always entertaining to count the number of men who came out of the women’s dorm, and the number of women who came out of the men’s dorm.
Speaking of dorm hijinks, one could “penny in” someone in a room. We had heavy wooden doors in steel frames but here was a little give if you pushed on the door at, say, chest level. Pennies could be placed in this gap, stacked on edge, if you get my drift. You’d jam them in there so tightly that it was very hard to turn the knob with the latch bolt force up against the strike plate.
My freshman dorm was the first co-ed one on campus. Two of three floors were every other room male or female (two to a room). The floors were known as “Sin City” but it was pretty boring as far as orgies and sex in the hallways go. As in, it never happened. Booze and pot? Not permitted but a daily occurrence. Just like the single sex dorms, I guess. Ahhh, the good old days.
I was once the victim of a classic dorm room prank. The prankster filled up the bottom of a paper grocery bag (remember those?) with shaving cream. He flattened the top of the bag and slid it under my door. Then he stomped on the full end with both feet. Shaving cream shot most of the way across the floor inside the room. I saw the bag slide under the door, and I had a few seconds to try to remember the way to make the trick backfire. It wasn’t long enough. I thought about setting the bag on fire, but that probably would have engulfed the whole room in flaming shave cream.
So what is the way to backfire that trick?
I never remembered how stop the prank. What I actually did was to dive across the room and slap both hands on the flattened bag. That didn’t work, and it may have actually made it squirt farther.
Heh, one of my favorite zombie apocalypse e-book series described that scenario. Oh, the author’s name is Mark Tufo… his protagonist’s name is Mike 
My youngest is in college and she and her room mate might have a system. I don’t know. She’s busy becoming a nurse and he’s busy becoming the next Bernie Sanders. They’ve been BFFs for years now.