Does plateau exist?
It has been experienced by many language learners and brief googling is bound to produce many articles on the subject, e.g., this one, this one, or this one. I think that the answer by @Tsundoku also confirms that it has been recognized by the experts. Plateau effect has been also described as a more general phenomenon. Below I share a few of my own observations and thoughts on why the plateau happens in language learning and possible strategies for dealing with it.
Most language learners that I have met experienced plateau around around B1 level on CEFRL scale - particularly struggling to lift themselves from B1 to B2. In layman terms it is characterized by knowing the main language structures, having enough vocabulary, and being fluent enough to deal with most everyday situations in the target language... yet, being unable to have an intelligent extended conversation in this language. Note that in many European countries B1 is a sufficient requirement for obtaining permanent residency or citizenship (if you are coming from outside of the EU).
There are some objective reasons why it happens:
- Vocabulary acquisition usually lags behind learning grammar. Language grammar is a finite (and typically rather small) set of rules that can be learned rather quickly, and requires mastering only a small vocabulary - just enough to be able to form sentences with the grammatical structures that one learns. (Though grammar has bad name among some language learners, one could argue that these simply prefer to learn it intuitively rather than using their analytic capacities.) The learner finds themselves in a situation where they have seemingly learned most of what there was to learn, but they still don't know the language - this is the point where the language courses typically start having word advanced in their titles... but the learner does not feel that they are advancing at all.
- In terms of complexity of different stages CEFRL framework is actually based on a logarithmic scale. This is most readily seen by the size of vocabulary required for different stages: A1 = 500 words, A2 = 1000 words, B1 = 2000 words, B2 = 4000 words, and so on (the precise numbers vary depending on the language). The amount of material to learn doubles at every stage (i.e., grows exponentially), but your learning (at best) proceeds at the same pace: that is, if it took you three months to get to level A1, you might need about four years to get to B2. Thus, if the learner quickly passed through A1, A2 and B1, it may seem to them rather unexpected that they are struggling to achieve B2 - either because they do not realize the size of the task or because they are depressed by its size. Notably, it kills the enthusiasm/motivation that one might have obtained from passing quickly through the initial stages.
Working through the plateau
New challenges require new methods As it is clear from the first bullet in the previous section, one needs to focus on learning different things and learning them differently. Notably, if diligently doing the class and home work was enough for getting to B1, it is no more sufficient after that. Although advanced classes sometimes exist up to C2 level, from B1 onward they play an auxiliary role - the learning must happen elsewhere: through reading, watching films, communicating with native speakers, etc. Some teachers at this point adopt unorthodox approaches: e.g., focusing on exposing their students to local culture and politics, in order to motivate their linguistic curiosity outside of the class. Some go as far as transforming their class in a creative writing workshop or simply suggesting students to enroll in a course taught in the target language (especially good for humanity majors, but less relevant to sciences).
Stable routines The sheer size of the task requires discipline or some other way of imposing constant use/exercise of the target language - enthusiasm usually does not last long enough to get through the plateau. This is where immersion is most effective, as one is fully equipped for benefiting from exposure to the language. For conversational skills it is useful to have one's life partner, roommate, or colleagues speaking the target language. Those not having access to native speakers, may try binge-watching movies/series or reading junk literature (which is usually easy enough to understand and follow through, unlike serious literary or scientific works.)
As the other answer points out, a plateau may occur at different levels of language learning. In immigrant communities it is not uncommon to have the language at B2+ level, permitting to have normal professional life in the country of residence. These immigrants are however often reluctant to read, watch films, or follow news in the language, opting for equivalent resources in their native tongue. This produces a curious phenomenon of people living for decades in one country, but seeming better informed and passionate about something happening in their homeland, thousands miles away. However, such choices are not a matter of purely linguistic needs and shortcomings, as they reflect one's cultural identity, need to belong to a community, etc.