Both http://tux.rainside.sk/gentoo/ ftp://tux.rainside.sk/gentoo/ are empty. Is it possible to remove them from the mirror list and from the website https://gentoo.org/downloads/mirrors/#SK ?
No, ftps://tux.rainside.sk/gentoo/ is alive and well; I have been using this mirror for years. In my humble opinion, it is the best and the most reliable Gentoo mirror provider in Slovakia.
WORKSFORME
Looks fine to me.