There used to be a feature on Special:MobileOptions that allowed blocking all images from being loaded. It was removed by user Konfused-Kitten, apparently here:
https://phabricator.wikimedia.org/rEMFR2cefc0fade9c42c22902074e6fec05a001b12578

The rationale was that this feature (1) was "barely used", and (2) obsolete, due to "lazy loading images".

However, the commit does not reference any data that support claim (1). In fact, using this option is one of the easiest suggested methods of disabling images suggested on English Wikipedia:
https://en.wikipedia.org/wiki/Help:Options_to_hide_an_image

All other methods require the user to have a high level of technical proficiency (e.g., creating useraccount and using user scripts, setting up greasemonkey script, using custom Adblock rules, setting up and configuring a local proxy) or involve blocking resources other than Wikipedia (e.g., configuring the browser to disable images completely).

Concerning (2), load speed may not be the only reason for turning off images. Some users have personal preferences not to see images when they browse, for instance, certain topics on Wikipedia. To quote the above page: "Some people do not wish to see some images on Wikipedia for various reasons – images may not be suitable for a work environment; they may wish to prevent their children from seeing such images; their religion may forbid it; and so on."

Note that the proposal of this bug report is NOT to implement some kind of personal image filter, as has been proposed in the past. The Wikimedia community's rightful opposition of such filters was based on the arbitrary categorization of images into filter lists, and the lack of neutral and objective criteria for such.

However, an option that allows users to quickly switch between all images being turned on or off would bring a lot of the benefits to those concerned about seeing certain images, while being entirely neutral about what is displayed.