When editing on the mobile website, there should be a checkbox to mark the edit as minor just like on the desktop version.
|Open||None||T158181 Aim for workflow equivalence for MediaWiki on desktop and mobile web|
|Open||None||T123694 Allow mobile web edits to be marked as minor|
Note that it is possible to mark edits as minor using Mediawiki API sandbox, visual editor, and other custom editors (such as WIKI+). The problem is only with the mobile wikitext editor.
Agree with Sdkb. This and also the ability to edit or at least see the /* section */ part of the edit summary. See T234982: Varying approaches to section names in edit summaries (mobile vs desktop and visual vs wikitext). Mentioning it because it could be part of the same redesign (add checkbox for minor, add textbox for section).