Safari has a bug with client-side generated downloads. For that reason custom builds for Safari were disabled for a while. However, because the next version is solving the problem (Safari nightly should already work) we've decided to remove browser-sniffing code entirely. This also resulted in huge simplification of the UI code.
Fortunately, the problem only affects custom ZIP downloads. @Obsyd I'm sure they never worked for Safari, the customization interface was completely disabled. You could only download the full release in a pre-generated ZIP file.
If you still wish to download a bundle in ZIP form via Safari, the only option is to download it from GitHub