Hi, EKW, and welcome to the G.E,
I came across this sentence:
In January 2013, President Raul Castro ended a five-decades-long policy on limiting international travel for Cuban citizens.
Is that sentence grammatically correct?
No, it isn't. It should be 'decade' in the singular form, (not decades). It is a compound adjective here. David and Gustavo have asserted this point already. See here: