until by the 18th century a Polish serf could be obliged to work 7 days a week for his lord
Not really... up to 7 work days per household, which usually meant that a family made of several people worked for 1 day a week, they also could pay someone else to do this job, and all of that was instead of paying taxes... today paying ZUS or paying off mortgage is so cool ? The same shyt with better makeup.
and could be raped or murdered at will by his master until the very end of the 18th century.
Mainly according to commie era stories about bad bad pre-commie Poland... It was clearly a stupid system but not as terrible as you believe, very much different than negro slavery. Besides, in the USA negro people were often treated like crap till 60's/70's of 20th century. But anyway, they for sure complain too much about it, slavery has nothing to do with status of young negros these days.
does this negative legacy exist in Poland in any form?