This has been raised a few times before (e.g #257) and it seems that it's been rejected due to better ways to solve the root problem in each case.

My root problem: outside of the ocaml community, it's fairly common to build outside of the source directory, and this should not modify the source directory at all; for example, this is the default expectation of debian's package building scripts

Example workflow:

$ cd $SOURCE_DIR
$ DUNE_NO_MERLIN=true DUNE_BUILD_DIR="$(mktemp -d)" dune build foo.exe
$ git status --ignored
Current behavior: lots of .merlin files added
Desired behavior: absolutely no changes to SOURCE_DIR, even ones ignored by source control system
$

People actively working on the project would presumably want the .merlin files, but packaging builds do not; this means that committing a change to the configuration files is not a suitable option.