Port Health is a report on what the buildbots have built.
buildbots are the machines that Macports runs and build binaries from the port files so that you can download the binaries rather than have to compile the source yourself
The web page shows a tick or cross whether the build worked or not. There are entries with a question mark ?
. These are where no build has been attempted yet. This is most obvious with Ventura, as of 29/11/02 there is no arm builder for this but there is a recent x86_64 builder, go ? for all arm builds and older Intel builds but later have a tick or cross. ? also shows up for some Leopard cases.
So in this case it tells you that the build worked for all macOS/OSX from High Sierra onwards.
The links from each OS name go to the fuller report that the buildbots make, however these logs are deleted after a time and so as the mysql8 portfiles and source have not changed recently then they have been deleted. The builds were made in October/November last year from here
The red mark for El Capitan says that the buildbots could not build it. Unfortunately as no recent change there is no log to show why. I would try to install it, as there is no binary macports will try to build it and your local logs will show what the issue is. This can be reported to trac.macports.org You might get lucky and someone be able to fix it.