https://www.mediawiki.org/wiki/VisualEditor/Gadgets/Creating_a_custom_command?veaction=editsource has some TAB characters in the code. These are displayed as arrows. IMHO this is confusing. In many places, tabs are replaced by spaces, so the problem is really minor, but as you can see, there are ways to put tabs in the wikitext.
I'm not sure what you're asking? You are correct that wikitext allows tabs (though they're pretty rare as it's hard to enter one). We intentionally show these with a special glyph (since 2012, I think) to make clear that they're not just one or more spaces. (In visual mode, we also do this for ineffective newlines which don't cause a paragraph break.) Do you think we should reverse this decision? It would make it harder to spot this and fix it.
On a related point, when I copied and pasted a long list, all of the newlines were replaced by little carriage-return arrows like ↵ and all the content was run onto a single line. When it saved, Parsoid fixed everything (thank you, Parsing team!), but it was quite impossible to read, and I nearly discarded the changes to avoid corrupting the page.