Sadly that’s not what happened
In a normal developpement cycle, you call a “features freeze” at some time.
It mean that the main product (Gutenberg) won’t have breaking changes after that.
Release Candidates version are meant to fix bugs only. During this time, Developpers can make theirs updates but :
– WordPress let only few days/weeks)
– They didn’t let time for plugin developers because the absolutely want to be ready for the big annual meeting (WCUS)
– The docs weren’t ready (they totally changed last week, resulting in a lot of 404 errors) so it’s really hard for devs to find what they need
Plugins like ACF are trying to be compliant since this summer. But there was so much changes in the core, and things missing.
WPML, Polylang are big plugin deeply impacting the admin area, so they need at least one month to be fully compliant.
ACF found a bug that won’t be fixed in 5.0 (we have to wait 5.0.1) so those who upgrade will see weird UI until the next release