Do you leave your air conditioner in the window during the winter?

Do you leave your air conditioner in the window during the winter?

This year I've been having issues with the air conditioner I bought last summer and I've been going crazy trying to figure out the issue. It will only cool a small space in my apartment to about 66 or 67 while on full blast, when it used to cover everything last summer.
After weeks of trying to figure it out, I was having a talk with my landlord who looked at me like I was crazy when I told him that I left it in during the winter. I sealed it from the inside so there wasn't any draft coming in, but he said leaving it in the window could cause damage to the AC.
Growing up, my parents left the same air conditioner in the window for 20 years and it hasn't had a single issue. After googling it (to make sure I'm not an idiot), some places like NYC recommend you leave your AC in your window for the winter.
Do you leave your AC in year-round?