As far as I can tell, just text. 
Discussion
Ok. My first test wrongly used TLV type 1, which is why it just looks like a long nostr url with no meaning.
My second test used the correct TLV type 0, which is why it pulled out the relay URL. Looks like only the first URL is rendered. (And I don't know why nrelay1 is TLV, I presumed you could add multiple URLs).
So Damus supports it, to a degree.