There is no legacy OS as when you start it it simply starts running the program. You just need to program it on a computer, but the software suite is always updated. It is ugly indeed, but not broken.
You can program it in simatic manager- which can convert other languages (KOP, AWL (Assembly-equivalent if i recall)) into FUP.
Which promotes using it for larger scale projects. Which will not have objects. Not have functions. Just kilometres of ladder logic, by the time the disaster management crew is called.