Typically a rowstore index is a B-tree data structure (on a column) that points to data pages in a rowstore dataset.

It's not technically columnar as such.

You can sort with a rowstore index today, but I imagine you have to materialize the index in some way to take advantage of vectorization.

I don't know about "most", but here is a list: https://en.wikipedia.org/wiki/List_of_column-oriented_DBMSes

I mean, I guess you could call a B-Tree over a subset of columns "columnar"... And bit-map indexes are by columns too — but that's really bending the definition of what most people mean by columnar storage in databases.

It actually is not.