A PR was submitted to add 1.3 support a while back, but it wasn’t officially reviewed and the author dropped it.
However, even if not perfect, it was a reasonable PR. So when a client recently approached us about Open edX integration with Canvas, we offered to implement LTI 1.3 support on top of content libraries (aka Libraries v2, aka blockstore libraries) using the lti1p3 library the aforementioned PR introduced. Here’s the proposed ADR, and the current state of the implementation.
I know this doesn’t really answer your question, @sambapete, as edX has not yet reviewed, let alone accept, this work. But at least you can rest assured that you’re not the only one worried about it. 