Is it really true that Poland hasn't shown up in canon since march of 2015.... is this really where our world has come to? Please tell me I'm wrong....
Unfortunately due to Hetalia having such a large cast, there are a lot of characters that have been neglected for a good few years. Poland is not alone.
I made a list of last appearances for minor characters, and these are the ones that haven’t appeared since mid-2015. Also these dates are being based off their last canon manga strip appearance, not random illustrations, since those aren’t specifically attached to a canon story. So some have made a couple later illustration appearances. The dates on older material may be off a little, but I did my best. Feel free to correct me if I missed something.
This is not including characters that haven’t been formally introduced, fully designed, or those that may not be canon anymore, like Tibet who hasn’t appeared since 7/23/2006 (The Story about the Early Days of China and Japan) and was replaced by a panda in the anime.