If you don't have any map packs installed, Atlas opens a dialog where one of the buttons is labelled Open Addon List. On retail, that button doesn't do anything. On Classic Era, it opens the Addon List like it should.

Looking at https://www.townlong-yak.com/framexml/live/Blizzard_AddOnList/AddonList.lua, AddonList_Show seems to be gone. ShowUIPanel(AddonList) seems like it would be an acceptable alternative, but that pops up the Addon List when the dialog shows, not when you click the button. Not sure what the fix is right now.