In theory the classes you took should be good forever, and the program would never change. If this were strictly true, then you could take classes for years, a few at a time and with frequent long breaks and still obtain your degree.
Most schools consider students to be students from admission onwards, and they never take them out of the system.
However, degree programs change, requirements change, and the schools must change their curriculum. While they grandfather in older, possibly out of date classes, and older programs, they have to limit them, and the easiest way is by time.
If you start a bachelor's program you can expect to be able to follow it without issue for 4-6 years without change even if the program changes for newer incoming students. If you change degrees you'll have to adopt a new program.
However, if you drop out of school, the school doesn't think of you as a person who will never return. But if the programs and classes are changing they will remind you, even if you haven't attended for some time, that in order to fit under the old program you'll have to complete it by a certain date.
So this is why they've sent you a letter. You are, in their eyes, still a student, eligible for a degree in a specific program, and they want you to be aware that even though you haven't been in some time, if you had intended to complete your education with the program you started then you will have to do so by a deadline.
Not all schools send out such letters.