In this case, you do user research with mock-ups and other extremely low effort prototypes. But the work done in that regard is the same for agile and waterfall

In this case, you do user research with mock-ups and other extremely low effort prototypes. But the work done in that regard is the same for agile and waterfall. If you truly do have a project where the MVP will take a year, and the prototypes would be useless until that year is up, then it probably doesn’t matter which methodology you use in that first year.