Here is a reasonable explanation.
Note, you can check out web sites withe a tool on this site.
Linux systems in the past two years ( at least ) have been shipping with IPv6 running.
Home routers are another problem. If you are renting it from your ISP, I would think the ISP will upgrade them when necessary. If you own it, you will find some sites you can not get to, when they drop IPv4 support.