What changed when search stopped matching strings
Two Google updates mark the shift. BERT (2019) improved handling of prepositions and word order — the small words that decide whether "visa for Brazil from Turkey" is about travelling one way or the other. MUM (2021) extended that across languages and formats.
The practical consequence for writing is that the exact-match keyword stopped being necessary and repetition stopped being useful. A page can rank for a phrase it never contains, because the system resolved the meaning rather than the string.
What replaced keyword optimisation is coverage: whether a page addresses the question and the questions around it. That is harder to game and easier to do honestly, which is roughly the point.