That looks to be an issue with how the validation has been programmed on the website more than Monzo itself not being accepted, seeing as it’s only highlighting the 00 value. Would imagine their web team need to adjust that.
There are online services they can use to verify bank details exist so refusing 00 as an invalid value isn’t necessary. They can probably just enter it manually over the phone. So seems more like a website issue than an actual acceptance issue.
Fair point and a similar conclusion to what I had reached, however, the Wiki page states TalkTalk accepts on the web form and, due to this weird validation of sort code, it actually does not accept it through the web form.
I completely agree though that this should be an easy fix for TalkTalk and is rather frustrating the way they’ve handled it!