Google Chrome will no longer support HTTP/2 on vanilla 14.04 after May 15th , even if you're using the latest official upstream NGINX packages. This is because 14.04 ships with a version of OpenSSL that does not support the ALPN extension (prior to OpenSSL 1.0.2 you're limited to NPN, now deprecated). There was a bit of back-and-forth about the exact date, as the change was originally scheduled for earlier. However, Chrome decided to specifically push back the date so that there would be an Ubuntu LTS release available with the required support . If you're still stuck on SPDY, that's going to be dropped too, so there's really no good reason not to simply use HTTP/2 at this point.
The ppa notes there's a newer version also
nginx recommends using mainline over stable: "We recommend that in general you deploy the NGINX mainline branch at all times." 
The stable branch will fork from the mainline branch shortly. The version shipped in 16.04 is very close to what the stable will be, because the fork hadn't taken place before 16.04's release. I expect there to be very few changes, which is why (as someone else pointed out) we expect to update 16.04 to the stable branch as soon as it is available.
> The ppa notes there's a newer version also
That's just noting that the version released in 16.04 is newer than the version provided in the PPA.
> Nginx's own development PPA
Actually it's a PPA maintained by a team that care about Nginx's availability in Ubuntu. In this case, the uploads to that PPA were made by the very same person who looks after the official Ubuntu Nginx packages available to Ubuntu users by default.
Is this outdated or not applicable to servers?
Either way, personally I would never upgrade a server in place these days. Treat your servers like cattle not pets: Rebuild from new base image, validate, put into LB/proxy, terminate old stack.
Literally the easiest upgrade ever.
However, you can also "upgrade" your stack by building a new image using 16.04 from scratch, and that doesn't need to wait until 16.04.1.
http://caniuse.com/#feat=spdy (77.39% global)
http://caniuse.com/#feat=http2 (70.15% global)
Doe this mean 14.04 with Apache 2.2 is affected? Their blog doesnt explain and leave plenty of people confused...
Even in cases where a HTTP/2 or SPDY connection will no longer be established for Chrome users, the browser will fall back to HTTP 1.1. Unless you're using specific HTTP/2 features, the main impact will be decreased performance.