(If you don’t care about the details of UTF-8 error handling, it’s safe to stop reading.)
In reference to https://hsivonen.fi/broken-utf-8/ , I think it would be appropriate to submit that post to the Unicode Consortium with a cover note asking the Unicode Technical Committee to revert their decision to change the preferred UTF-8 error handling for Unicode 11 and to retract the action item to draft corresponding new text for Unicode 11 for reasons given in the post.
I think it would be preferable to do this via Mozilla’s liaison membership of the Unicode Consortium rather than me doing it as a random member of the public, because submission via Mozilla’s liaison membership allows for visibility into the process and opportunity for follow-up whereas if I do it on my own, it’s basically a matter of dropping a note into a one-way black box. (It seems that this kind of thing is exactly what Mozilla’s liaison membership is for.)
However, submitting via Mozilla’s liaison membership raises the question of whether the submission would properly represent a Mozilla consensus. I estimate this to be noncontroversial, because deliberate effort has been expended to make the Mozilla-affiliated implementations that I am aware of (uconv, encoding_rs and the Rust standard library) behave according to the pre-Unicode 11 version of the guidance either directly by looking at the Unicode Standard or by the way of implementing the WHATWG Encoding Standard, which elevates the pre-Unicode 11 preferred approach into a requirement.
If I have mis-guessed that the above-contemplated submission should be non-controversial from the Mozilla perspective and you believe that the above-contemplated submission should not be made via Mozilla’s liaison membership, please let me know.
(My understanding is that a reversal of the decision is quite possible, but actually making the above-contemplated submission is a process prerequisite for a reversal to take place.)