<a href="https://en.wikipedia.org/wiki/Atiyah%E2%80%93Singer_index_theorem">Index theorem</a> of Atiyah and Singer closed a substantial field of research in the 1960s. I knew people who were working in this field, and had to switch the field of their research
completely.

It also happens sometimes that a new breakthrough does not really close the filed, but many people have to switch to another field because they are not equipped to understand the breakthrough.  But I do not want to give examples of such a sad situation.