The most recent copyright on the ICCCM is 1994.
Sometime in 2002, a window manager may have, for the first time, full ICCCM 2.0 compliance. Kudos to FVWM.

Of the window managers in Debian Sid, only the most recent FVWM has not yet failed simple compliance tests. These window managers include twm and two versions of mwm. The non-free version of mwm showed the most promise, but ultimately failed.

It's been 8 years. Only one window manager (out of very very many) may be fully compliant. Meanwhile, selection managers in general have poor compliance. The standard xclipboard program in XFree86 4.1.x does not comply. The recent XSETTINGS spec at freedesktop.org is not compliant by design; compliance is seen as too difficult. The system tray spec as originally proposed was compliant, but the current proposal is not. Note that neither of these specs forbids an implementation from being compliant; both use a 'managed' selection without target support.

8 years. What does it take to change the ICCCM?