If a client doesn't support these, do they still show up in people's feed? š 
Yes, the UX is a bit clunky and hidden now because Iām still experimenting with it but you can comment on any url with Nostur. Right now you have to paste the URL in search to do it.
Hopefully other clients will adopt it too and you could also comment from the browser eventually with something like nostr:npub1alpha9l6f7kk08jxfdaxrpqqnd7vwcz6e6cvtattgexjhxr2vrcqk86dsn browser extension: nostr:note1vcr77y8m4gz4qzusczyq8nrvlew56dlpt0czax4g8d6reaw5fpuspzhtje
Discussion
On Damus they look like a reply to yourself š
lol don't mind me, I'm just out here talking to myself šš
The comments are regular kind 1 so most clients should be able to show the comments, but it requires some special handling to make it appear as a comment on a website instead of reply to a post.