I also prefer ATX headings.

On Thursday, 3 September 2020 17:57:59 UTC+2, John MacFarlane wrote:

Currently in writing markdown (and commonmark) pandoc emits
Setext-style (underlined) headings for levels 1 and 2, ATX_style
(#) for the rest, unless the `--atx-headers` option is used
to force ATX.

Proposal: given that setext headings are becoming more
uncommon, and in view of the ugliness of switching heading
styles at level 3, default instead to ATX-style headings,
unless a new `--setext-headings` option is used.

Comments?

Note: in principle we could also change the default just
for commonmark/gfm output, not for default markdown.
However, this would still require adding a --setext-headings
option.