This is intentional. This was discussed in https://phabricator.wikimedia.org/T319449#8336068.

This was again considered in T326887 and other related changes which were made due to feedback that the toggle should always be accessible regardless of the page you are on.

Eventually the hope is that this becomes generalized to provide other kinds of settings such as dark mode and font sizing (see T91201).

Those discussions appear to focus on displaying it regardless of page width, rather than discussing displaying it on pages where it is disabled. On the latter I don't think it is a good idea, as non-functional buttons are likely to confuse users.

The limited width feature works on https://en.wikipedia.org/wiki/Category:Astronomical_objects_known_since_antiquity and other pages where maximum width is disabled by default (see screencast below). Before the community request of T326887 this button would have been hidden at lower resolutions where it wasn't applicable. Perhaps you are have a gadget enabled that is interfering with its functionality or are using a browser window size which is causing you confusion?