ISO 8601 contains way too many obscure formats. RFC 3339 is pretty much a subset and defines only sensible ones. It also allows 2018-02-05 08:02:43-00:00 (no T and explicitly specifying no timezone)
What PR? And what about the missing API that satisfies every/most desktops' needs?
And any GNOME user who needs that can use the extension. I don't really get the point, apart from philosophy, which doesn't really make sense here since nothing perfect exists yet, which GNOME seemingly doesn't like implementing. Maybe some work towards that would be good, but I'm just someone using software for free, without paying anything.
As is, the current GNOME is unusable to me without extensions because they refuse to implement support for appindicators. You literally cannot use applications that minimize to tray on vanilla GNOME right now.
They have been talking about adding their own protocol for years but that is of no use when things are broken right now.
So what, just use the extension. Currently no cross-desktop API for systrays that doesn't suck in one or another way exists, so GNOME doesn't have support for them. If you care that much about not using an extension, implement it for yourself.