I don’t think the person that responded understood the poster was looking for the place to create custom columns, and so their answer was correct within the narrower case of simply wanting to toggle available columns (not make new ones). Custom columns can of course be toggled from the main menu, as described, or by right-clicking too. They were not describing a bug, they just did not know how to find the configuration settings.
There are two conveniences that lead to Project ▸ Project Settings..., Custom Metadata tab: the missing button, which will be fixed in the next update, and the Inspector metadata tab itself, where one can click the ••• button in the middle section, but I don’t recall there ever being one from the main View menu.