Technically yes it's very easy to bind the server only to IPv6.
Most systems (maybe 70% today) couldn't reach it, it's harder to estimate how many _people_ because people may have many options to access the Network and some are more likely than others to be IPv6 enabled.
It also varies geographically, in some places (e.g. Germany) it may be about half of people who have IPv6 access, in say Australia it might be more like a quarter and so 75% could not reach that site.
As we see above in this HN thread some users intentionally disable IPv6.
Yes its possible, I run a website on 2 IPv6 only servers. Although I had to use cloudflare as gateway for IPv4 only users.
If I were to remove cloudflare, more than 30% of users can connect to the site via IPv6. Problem is that other 70% will consider the site dead and move on.
IPv6-only hosting typically includes a shared IPv4 reverse proxy for backwards compatibility, eg. https://www.mythic-beasts.com/support/topics/proxy (Mythic Beasts host the Raspberry Pi website; I am a friend and customer.)
There's a problem that just having an IPv6 address is indistinguishable (for such users) from a server that's not configured or is down. We need a good solution to indicate to end users that the server is working just fine, but just you don't have IPv6 support and can't reach it because of that.
But I can't send anything to such a user directly if they don't support IPv6 and I don't have an IPv4 address.
Perhaps we need some community-run webservice with an explanatory page saying "Website [something.com] requires IPv6, here's lots of documentation on how to try and fix this" and everyone could point their domain name IPv4 address to that webservice, and the IPv6 address to the real server.
Well, because they (and their entire organization) may be unable to get a publicly routable IPv4 address, because NCC can't and won't assign any more. You can work around it for now, but in the future this will be a more and more common situation.
It means they can’t get one for free, not that they can’t get one at all. IPv4 addresses are trading for around 20/each when bought in blocks of 256. They won’t be prohibitively expensive for any serious hosting provider for the foreseeable future.
For hobby projects hosted on a Raspberry Pi in someone’s bedroom, I can imagine a static IPv4 being prohibitively expensive, but that’s about it.
What you're probably going to see is increasing cost to keep a dedicated public IPv4 address. If you don't, you will probably either have to have IPv6 or NAT.
Residential will probably lose public IPv4 addresses faster than for hosting services.
