anyone know if I can use a 9spd xt cassette on a latest ultegra 10 spd hub? The blurb says 10 spd only, is it true?

Run a spacer on the old one so can’t see why not

im guessing here, but:
probably needs a spacer, as the freehub may be a bit wider, but i think you should be ok?
if you have an old cassette, break it up and take the spacers out of inbetween the cogs?

as long as the shifter matchest the casette, you should be ok.

I think shimano 8/9/10sp are all technically the same width freehub, but ultegra/dura-ace freehubs are probably alloy rather than steel. So you’d probably want to fit a cassette with an alloy carrier rather than a cheaper all steel one.

The hub may actually be smaller. There are a few hubs out there which are 10speed only, but they seemed to realise that this wasn’t a great idea. Certainly a spacer isn’t going to help, people are getting confused between the change from 7 to 8 speed.

A 9 speed cassette is 1mm wider than a 10 speed specificfreehub body. You would use a spacer to put a 10 cassette on a 9 body. Current 6600 rear hubs are 9 amd 10 speed compatible but I am pretty sure early ones were not. However 6600 pre-built wheels are 10 speed only.

Current 6600 rear hubs are 9 amd 10 speed compatible but I am pretty sure early ones were not.

Ultegra hubs (as opposed to wheels) have always been compatible with 8/9 – it’s just DA ones which weren’t (though I think the latest iteration of those are also).

I thought DA 10 speed was a deeper notch as well, so a DA cassette will fit on another hub, but only a DA cassette will fit on a DA hub.

For the original Q, yes, you should be fine as long as there was a spacer behind the 10s cassette.