You’re welcome 
And thank you for using Jekyll and sticking with it for years. 
Even though there has been a change in existing behavior, I’m not sure if there is compelling reason to revert some of the optimizations made (and honestly, I can’t pinpoint which change exactly) and release a v4.2.1.
@nfrankel A note of caution with respect to the gist you linked to, if one of your plugins implement a use for the :post_init hook for the generated pages, then there could be unexpected behavior because instead of triggering the hook at the end of initializing, it could get triggered just after the super calls.
What you could do is define a private method for the hook in the parent class and then reference that method at the end of the initialize method…
I made it working so for me, I don’t see any reason to revert.
I’d just advise caution about making changes in the class hierarchy in minor version releases next time 
I understand that it’s much easier in statically-typed languages but still.
Note, I’ll remove the hook altogether as I don’t them - it was plain cargo culting…