It looks like jaunty and karmic were using these to transition people from 5.0 to 5.1. According to Ubuntu's changelogs these packages were mostly backports from Debian unstable, since Debian stable is still using 5.0. There's a lot of stuff in their changelogs and a lot of noise on various packages' bugtrackers from around that time about the change from libmysqlclient15 -> libmysqlclient16, which looks like it didn't get completed until lucid.
It looks like jaunty and karmic were using these to transition people from 5.0 to 5.1. According to Ubuntu's changelogs these packages were mostly backports from Debian unstable, since Debian stable is still using 5.0. There's a lot of stuff in their changelogs and a lot of noise on various packages' bugtrackers from around that time about the change from libmysqlclient15 -> libmysqlclient16, which looks like it didn't get completed until lucid.