They had to rename it to “MI-5” for the US market. The word doesn’t have the same connotations in the US as it does in the UK. One reason is that it was used as a racial slur in WWII and after the war.
Not what you are doing, and I understand that's why they renamed the show but they shouldn't have. While it can be used as a racial slur it doesn't generally have that connotation. I think if you directly asked most people they would either not know about the slur or say something like "I think its a racist term for some group or something." I had to look it up myself to be sure. We can choose to let that usage of the term die.