A lot of folks have been asking themselves why the outstanding Advanced Custom Fields plugin is still not baked into WP Core. Seeing the number of installs (1+ Mio) it certainly is not a wrong question to ask.
Out of backward compatiblity, and just easy of use, there should be an off-switch, at best supplied as a simple auto-load option (eg. 'acf_in_core' or 'acf_base_integration' or something like that). This would also ensure that future massive updates to the original ACF plugin may not collide with "our" core version, plus it'd still enable you to properly use ACF Pro ;)
I'd also suggest keeping the ACF infrastructure (field groups CPT / taxonomies etc.) the very same, so in the case of eg. ACF 6 or maybe even a custom version for ClassicPress, one indeed just uses the "off" switch (or the plugin installation hook checks for the enabled option and just disables it automatically during plugin activation).
Suggested version to integrate is 5, not 4. I release its not released yet, only available in a preview relese from the ACF site, but its better to have up-to-date structures in place, than slightly outdated ones. Also, most people who use the pro version are already on 5.x.
My guess is that the official release of the base ("free") version has been delayed thanks to a) the advend of Gutenborg and b) the overwhelmingly massive amount of reviews to be done by the Plugin Review Team (essentially it might just be stuck in the review pipeline).