Somewhat motivated by How to progress in rock climbing after grades v4 / 5.11+? I am curious if anyone has tried to document if there are particular grades that climbers plateau on and need more time to work through. In other words, do many climbers get stuck at V4 an need to work hard to progress and then again at V7, or is it an individual thing that some get stuck at V3, other V4 and some V5?
I can only answer from my personal perspective. It might not be statistically relevant, but I dare say it could be a little generalisable.
I have been climbing actively (more than 3 days a week), both indoors and outdoors, for the past 3+ years. I have climbed with beginners and was mentored by people that climb since the 1990s. Most of my climbing partners today have around 10 years of experience, and I know most of their trajectories within climbing from their stories. I have also taught climbing to 5 or 6 people and watched them blossom.
First of all, I strongly believe people "plateau" at a grade mostly because they have the wrong mindset. It might be possible that a large group of people usually get stuck at a certain grade, but by discussing this with everyone else, that grade ends up being mystified. Even people that wouldn't normally find those grades very hard end up being beaten by the majority - that was my case with 5.12. See my answer in the link the OP attached to his question for a more thorough development of these ideas.
Also, there is an extremely important age component. From my perspective this is very easy to identify. Beside the obvious metabolic component, people usually think that if you start climbing early, your body will have more ease to adapt to the harsh conditions climbing exposes you to - and that's very, very true, but there's more: children are not discouraged by the myths adult create around a grade. A children/teenager doesn't give a damn about 5.12 being "so hard" for so many people: they just go there and send. Everyone remembers how reckless we were when we were teenagers... This also leaks to climbing.
Now that the introduction has been duly made, we can cut to the business. People usually get stuck at the transitions between
V3 - V4 (mental)
V5 and above (real)
5.10d - 5.11a (mental)
5.12 and above (real)
I've split the transitions between the ones I believe are mental and the ones I believe are real. The mental ones are the mystified ones: they usually rest on a threshold and require a change of mindset. If you climb 5.10d and have a strong mind, you can certainly climb 5.11a. You just need to get used to the fact that you climb better than you think. This is not true, however, for the steps between sending V5 and sending V6. There's obviously a mindset tuning, but climbing a V5 does not guarantee you'll be able to send V6 at the same pace you moved from V4 to V5. You will be required to hangboard, campus, and practice other specific exercises that increase your tendon strength and anaerobic power/resistance. People that redpoint V4 have usually no idea of what if feels like to send a V7 - because it's impossible for them. Their body is not ready. They need specific training. I have seen people climb V3 on January and onsight V7 in July, but that's not normal. Those people are different, and are usually teenagers.
That being said, if you climb 5.11, you already have a good notion of what sport climbing is. You'll have a shock transition between 5.11 and 5.12, and then you'll be introduced to the world of specific training. From then on, you already know what you need to do: climb less, train more! The grades below could be achieved only by climbing, but the ones above 5.12+ usually (USUALLY!) need specific points that you can only develop by complementing your climbing with campusing, hangboarding, weightlifting, etc. The same applies for grades above V5.
Many people don't understand (or simple don't care) their body requires training to break some barriers, and that's why most people plateau at redpointing V4 and 5.11b/c. Some people are very light/strong/young/motivated and don't even notice those barriers, but most people I know, do. I also know a lot of people (myself included) that are stuck at the 5.13 transitions - but come on: those grades are meant to be plateaus. For most people, there is no quick pace after 5.12+. Training (not climbing!) at least 4 times a week is a must, and we know it.
I can't speak specifically to "indoor climbers", but i've definitely witnessed some fairly common overall plateaus, and experienced a few myself with regards to outdoor climbing. I would however say that it is VERY dependent on the person... and the ones that are common are typically in the mid-range grades, and are often "number" dependent... meaning they are related to people mythic-ally associating a number with whether or not they can climb that grade.
5.13c/d seems to be a common plateau for a lot of people. For whatever reason, this is the hardest that a lot of people ever climb. It's a bit of a make or break grade for people who break past the 5.12's. I believe that the reason is that progressing into 5.14 requires either extremely rare genetics, or an extremely specific training regime that a lot of people simply can't commit to due to life/time/mentality limitations.
5.12a/b seems to be another plateau for a lot of people. Either climbing 5.12 (which for many, ends up being a life long or multi-year goal) or progressing beyond 5.12a. Many people finally climb 5.12a and then stagnate.
5.11a also seems to be a big hurdle for a lot of people, which I believe mostly to be mental more than physical. I think physically, just about anyone can climb 5.11a, but mentally it's just this huge blocker that people can't seem to be able to bring themselves to be able to do.