Our original messaging here was largely just lifted from the equivalent
message for unknown values in "count", and it didn't really include any
specific advice on how to update a configuration to make for_each valid,
instead focusing only on the workaround of using the -target planning
option.
It's tough to pack in a fully-actionable suggestion here since unknown
values in for_each keys tends to be a gnarly architectural problem rather
than a local quirk -- when data flows between modules it can sometimes be
unclear whether it'll end up being used in a context which allows unknown
values.
I did my best to summarize the advice we've been giving in community forum
though, in the hope that more people will be able to address this for
themselves without asking for help, until we're one day able to smooth
this out better with a mechanism such as "partial apply".
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
