what are the practical/real world cases is which this is useful?

most go modules are defined at the root directory of repos, and most people have go projects cloned onto their local filesystem either in one directory (flat layout) or in a hierarchy of sub-directories with a known/limited depth. So a -maxdepth flag can be used to limit the search scope in these cases and prevent bogus or unwanted go.mod files from becoming accidentally included in the work space (e.g. issue #51959) at which point users would then have to prune those out, which is cumbersome.

Depth seems like a poor proxy for the actual issues, a multi module repository can easily have modules at different depths, while a checked out dependency in the same workspace (eg, x/tools from the other issue) may have "unwanted" go.mods at the same depths.
I don't see this as a generally useful mechanism for excluding unwanted module declarations.

-maxdepth is only useful for cases where the desirable depth of directory for where to find go modules to be included in the work space is known. I suspect this is the common case.

Even for cases where a directory level contains both desirable go modules and undesirable ones, limiting the search scope with a max depth would result in fewer (or equal) undesirable ones that need to be pruned out than that of allowing the search scope to be unbound.

CC @bcmills @matloob

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

Because -r is not the default, another option is to use other file walking tools to prepare a list of directories passed to go work use. It seems like -r does not need all the bells and whistles.

Because -r is not the default, another option is to use other file walking tools to prepare a list of directories passed to go work use. It seems like -r does not need all the bells and whistles.

Yeah, I'm okay with that.

This proposal has been declined as retracted.
— rsc for the proposal review group