Wikitext editor toolbar and welcome dialog give the option to switch to visual editor even if it's disabled in user preferences (or not enabled, on opt-in wikis).
I am about 80% sure this is unintentional.
It's particularly awkward on opt-in wikis. First of all, the editor is usually opt-in for a reason (e.g. it doesn't work well on Wiktionary because the pages are composed almost entirely of templates), and new users who follow the welcome dialog's prompt to switch will have a bad time. Secondly, after switching the visual editor is still disabled in the user's preferences, so when the user edits another page, they will see the unfamiliar wikitext editor.
On opt-out wikis, I assume anyone who opted out is an experienced editor who will just switch back, but they might be upset and feel like we're forcing the visual editor on them if we offer them a prompt to switch.
(I found this after looking into T242514 "Disable VisualEditor on Polish Wiktionary" and finding that it's already supposed to be disabled in our configuration!)