The traditional Custom Fields in Wordpress have been around since the beginning, gives some degree of customability to the posts. They are very basic, but the work.
Soon after, the Advanced Custom Fields become the norm, adding a nice UI on top of the custom fields, and enabling things like checkboxes, time/date, repeater, etc.
All was great in Wordpressland, but the ACF were sold to WP Enginge who are now trying hard to monetize the ACFs. Instead of payment per installation, they now moved ACFs to a monthly/yearly subscription model.
I still keep using ACFs for client work, but try to remove it from my personal sites - like this one. All it did was to have a checkbox to enable/disable Markdown on the posts.
Markdown is now by default on, ACF no longer needed.
Also, I was stuggling to show the original Custom Fields:
Only AFTER I uninstalled ACF, the option showed up again!