Based on Voting done back in the SF forums:
These skins got no votes from users:
These two skins nearly identical to smalldefault (including font size):
A few skins that received no votes are still going to return such as some cool skins (since it was easy to update them all) and blue (since it is very large high contrast text which may be helpful to some people).
I began work on automatic detection of RTL languages that would @include a RTL specific CSS file so we could eliminate sleet-rtl while still making the skin work for RTL languages.
I think the simplest solution would be to have the installer silently delete these skins from existing installs. Then the PF code can deal with the case where the skin preference is set to a skin that does not exist and substitute the default skin.
Other skins are all up to date as of 2005 I believe.