Received from J_Catlady at 12/15/2018 01:20 AM UTC:
In any case, if the language is different for restricted vs. unrestricted groups is different for main groups, I think it should also be correspondingly different for subgroups. Users will get used to one or the other and assume "join this group" means the group is unrestricted. But let's get first consensus on whether or not this is true - as I said, it's definitely true for the restricted and unrestricted groups I checked.
And not true for the ones I checked.
Did you delete your Gio cookies before testing? Remember, the condition being discussed also included the case of a nonmember of the main group. If you tested on your own groups with cookies active you weren't a nonmember of the main group.
And in the end it makes no difference if "Apply" is used across the board because an "Apply" is approved immediately in the case of an unrestricted group, so the impression given to the new subscriber is, in effect, the same as the result from the "Join" that you prefer.
I see no good reason that Groups.io users should be lead to form the distinction you (incorrectly) said already exists.