More than Flushing (also Caching) for innodb_flush_method, and Missing Release Candidate

Something changed in MySQL 8.4 related to caching, and it is easy to miss, so it deserves a post. And a subject adjacent to this is the missing Release Candidate for MySQL 8.4 LTS, with my hope that the next LTS will have a Release Candidate, so I also cover this topic below. (if you are not interested in Caching and Flushing, you can jump directly to the section about Release Candidate)