"go tool api" exists for our own internal quality control. It is not a general tool for others to use. It is likely being shipped in the binary distributions just because it gets built like anything else and is needed for all.bash.
The binary distribution packager should remove the cmd/api binary
from the list of files actually packed up. Not having it will make clearer to users that they should not use it.
(Go 1 compatiblity says nothing about tools, so the fact that we shipped one before doesn't matter.)
See CL 15140 for an example of someone thinking it's a general tool.