anything that's 90% match should be good enough.

There's no such thing as a '90% match' when you are doing nearest-neighbor lookup. Percentages of what? There's just distance in a high-dimensional space, which is itself dependent on dimensionality and pretty arbitrary - different embeddings will have totally different 'units'.

You could solve this in the UI by telling them that they were redirected because the original page was not found.

That doesn't deal with issues like archiving or updating links to follow redirects which turn out to be spurious because you thought '90%' was a meaningful number.