They are very easy to add.
Easy if you don't want to make them super high quality. The concept of adding ironsights is relativly easy, but also incredibly tedious, especially when you're doing it with tons of weapons. I've personally experimented with flawless systems for the uses of ironsights, but it seems rather useless. Ironsights don't really offer anything worth adding into blockland combat. The whole idea of ironsights is to increase a players accuracy with said weapon while restricting their movement and FOV, but, seeing as how players can already zoom in with their F keys, it doesn't seem to have any practical use other than for pure visual nostalgia.
If you want to go to the trouble to add ironsights, be my guest, but they usually turn out looking ugly, and make combat more annoying.