> It would be even better for me to be able to say "sorry, I can't open docx files, please save it as odt", than always having to warn people that my changes might screw up the document.
Then just say it.
If OOXML support in LibreOffice is not good enough for you/your documents, then just do not use it and refuse OOXML documents. Do not bore your colleagues with your free software life and just tell them: "sorry, I can't open docx files, please save it as odt". They will thank you for this language approximation, way less extreme than trying to entirely remove the feature from the product!
Other people than you are happy with a "pretty good" only support. Yet other people are not happy but still want to see the feature to test it and report bugs and monitor its progress... generally speaking there are a number of well-known reasons to ship imperfect features; no need to elaborate here.
I think the only reasonable thing you can ask is a better labelling of the feature.