Your comments

I’m for this. I’ve written a novel using Textastic on iOS and I have auto-correct (read: auto-dement) turned off as it usually mis-corrects (ie dements) the text I write.  It gets harder when you’re writing science fiction and it wants to auto-correct alternate pronouns.

I’d love to have a “Underline Spelling Issues” setting that we can set for certain mark-up languages.  Maybe have the setting ask for what formats to activate on.  When a word is detected as misspelled, red-dot underline it, and let a tap on the word present suggestions (or give it an option to see the suggestions).