For column index based filtering it is important to have enough pages for a column. In case of a perfectly matching encoding for the suitable data it can happen that all of the values can be encoded in one page (e.g. a column of an ascending counter).
With this improvement we would be able to limit the pages by the maximum number of rows to be written in it so we would have enough pages for every column.
Based on the benchmarks listed here 20k seems to be a good choice for the default value.