https://gerrit.wikimedia.org/r/#/c/322138/ adds support for the autocomplete parameter in HTMLTextField but OOUI\TextInputWidget expects a boolean. It should instead transform boolean values to on/off but accept anything else as well (see https://html.spec.whatwg.org/multipage/forms.html#autofill for the full range).
IMO validating is not worth the effort (I didn't add any to HTMLTextField).
Note that textfields are autocompletable by default (or more precisely take their default from the parent form, which defaults to on). This attribute can be used to turn off or restrict that. (TBH I have no idea if it's useful or even has browser support. I just wanted to add support for autocomplete=off to HTMLTextField and figured there is no point in disallowing the other values described in the spec; then it was pointed out in CR that OOUI interprets the parameter differently.)