Obsolete skins have been moved to a "legacy" section. Most are orphans, i.e., the developer is no longer active (in some cases we have no idea where he is). Many do not work in the current version of jAlbum. Almost all have designs that are from an earlier era - they're non-responsive, have designs based on framesets (a disaster on any mobile device), and so on. There is a link to the legacy skins buried at the bottom of the list of old jAlbum versions. No one should even consider using any of these skins for a new project - many don't work at all. They are kept solely in case some user of an old jAlbum version needs a way to rescue an ancient album, though even in that case I would strongly encourage the user to update jAlbum and choose a new skin.
The list of current skins still contains some that I wouldn't advise using, but that's more a matter of personal preference than anything else. There are a few that are not responsive (either in part or in whole), but if you're not concerned about visitors with small mobile devices like smartphones, that's not a fatal flaw. Some are not actively supported, so bug fixes and new features are unlikely to happen.