They did away with private addressing (site-local) "because it breaks the openness of the internet and firewalls"
They did away with site-local addresses because they couldn't agree on the definition of a site (is your home network a site, or is it part of your ISP's network?). They've been replaced with ULAs, which are easier to understand, simpler to administer and simpler to program with.
The standard has changed so many times in the last 10 years
A few unused features have been removed (v4-compatible addressing, site-local addressing, partial support for MTUs below 1280), but the specification has been mostly stable for a good 15 years.
They did away with IPV4's simplistic subnetting and supernetting,
No, subnetting is still there. The only difference is that all leaf networks are supposed to have the same size, which is supposed to make administration easier.
introduced EUI-64 addressing
Yeah, that was a stupid idea. It's been replaced by RFC 7217 (stable private addresses).
Very Few large deployments.
Google? Facebook? Comcast?