Alexander,
All of our products, including REF5050, undergo a life-time test in a high temperature oven to determine an equivalent shift of various parameters after 10-years (87,600 hours) of constant operation at ambient temperature. Based on the above test, the expected long-term shift after 10 years of operation is equal or less to the maximum initial accuracy of 0.05% specified in the datasheet (500ppm).
Even though REF5050 may work well beyond our specified 10-year life, we do not have the data to directly answer your question about the expected long-term shift after 20 years (175,000 hours) but IF the part is still alive, the expected shift should be: 50ppm*√175=661ppm.
For more detailed explenation of the long-term shift, please read my blog under link below: