CoreELEC is a ‘Just enough OS’ Linux distribution based on Kodi1 technology for popular Amlogic hardware.
The legacy builds are what was the stable release of the previous major version (9.2.x) of CoreELEC. We realize that some users may not be able to use or even be imediatly interested in using the latest major release of CoreELEC, so we will maintain the legacy builds for a while so you can still easily find and install the previous 9.2.x builds.
Legacy builds will in general not recieve new features, or support for new devices. Development work on these builds will be very minimal, since the developers are now focused on the new major version of CoreELEC. These builds will however recieve some basic maintnance such as the ocasional bug fix, security updates, etc...
If you are interested in using the latest stable builds from the current CoreELEC release you are on the wrong page and will want to look here for the latest stable builds.