This change does impact git-annex config
eg "git annex config --set annex.addunlocked on"
will store "on" and new git-annex will understand that value, while
old git-annex will error:
git-annex: bad annex.addunlocked configuration in git annex config:
Parse failure: near "on"
That seems acceptable.
Not special remote configs that are only documented as =true or =false
however. Having git-annex support other values for those would break
backwards compatability when used with old versions of git-annex. And
older versions ignore invalid special remote configs.. That would not
be a good combination.