Mine was turned on by default, I just checked.

Did you upgrade from a previous version?

Maybe the new setting checks a certain combination of other settings first, like:

    if (setting_1 && setting_2 && !setting_3)
    {
        new_setting = true
    }

Mine was also enabled by default.

I have Photos shared library set up, one of the members of the library is a child account — I wonder if that’s got anything to do with it.

odd, i'm also on latest but it was enabled