@soulfire It is still straight up illegal to watch DVD's on GNU/Linux in the USA (in the USA it was determined that DeCSS violated the DMCA).
In the USA, thanks to the DMCA, any circumvention of "technical protection methods" is illegal (with very few exceptions that people need to fight for every 3 year to keep).
DeCSS still works now (including an implementation which was a then found largest prime number that happened to also work as a x86 .elf using Linux SYSCALLs that decoded CSS).
Typically, libdvdcss is now used instead, as it automatically generates (via numerous methods with a fallback to bruteforcing) all the illegal numbers needed for DeCSS for you.
As there are been no legal challenges to libdvdcss, it's assumed that it's a legal gray area, but it's not - it's just that if anyone tried to sue for the usage of libdvdcss in the USA, that would just force many people and businesses to work towards getting rid of the DMCA.

A bigger issue is probably playing blurays on GNU/Linux - bluray reading libraries exist, but you need to provide such libraries the necessary keys and there's a lot of them.