Some teachers maintain contact with former students they were particularly close to or who made a significant impact on them. Others might think about former students occasionally, especially those who went on to achieve notable things or who faced significant challenges. Many teachers, however, have so many students over the years that it's impossible to remember them all individually. The sheer volume makes it impractical to keep track of every graduated student's life.
The strength of the teacher-student relationship is the biggest factor. A strong bond might lead to lasting memories and occasional thoughts, while a more distant relationship will likely result in fewer, if any, lingering thoughts.