What important theorems were less well known before 2018? I mean results that could easily make into textbooks or research monographs, but almost nobody knows about them. If you provide an answer, please:
State only one theorem per answer. When people will vote on your answer they will vote on a particular theorem.
Provide a careful statement and all necessary definitions so that a well educated graduate student working in a related area would understand it.
Provide references to the original paper.
Provide references to more recent and related work.
Just make your answer useful so other people in the mathematical community can use it right away.
Add comments: how you discovered it, why it is important etc.
Please, make sure that your answer is written at least as carefully as mine. I did invest quite a lot of time writing my answers.
As an example I will provide three answers to this question. I discovered these results while searching for papers related to the questions I was working on. I will post two answers right away, but I need slightly more time to write the third one.
