And if all else fails, you can put something like Cloudflare in front of it to handle IPv4 traffic.

Which than you're back to paying $40+/year to ensure you don't get wiped from their "free" tier when they feel like it.

Nothing is free forever.

I've been on their free tier since launch. Granted, it's nothing high-traffic, but a personal website shouldn't reach any "please upgrade" thresholds.

They have had a free tier since they launched over a decade ago. I think they’ve found a way to monetize that traffic or at least the data they collect on the sites they proxy because it’s survived so long.

I think it's the same model as free antivirus. Free customers provide a lot of data to analyze and detect threats, which translates into increased value of the product to the paying customers.

Also gives you a lot of traffic which you can use to test new deployments without disrupting paying customers.

True, it is a temporary solution though to how long Cloudflare offers it.

Might be a good idea to make a hidden request from your homepage to an ipv6 only resource. Then log how many of your visitors actually fail.

It's a chicken and egg problem: as long as sites are available through ip4, ISPs have no incentive to provide ip6, and since ISPs often don't provide ip6, sites can't go ip6-only. One possible solution would be to provide both and throttle ip4 traffic, then better speed can provide incentive to upgrade to ip6.