That has been common knowledge since the fall of communism but here is just one of the many articles on it...
https://warsawinstitute.org/russian-population-declines-rapidly-kremlin-faces-demographic-crisis/
Yes, the collapse is quite remarkable. Look at Chukotka, the vast area of Russia across from Alaska. In 1990 it has 160,000 people. It now only has 50,000.
Regions like this will not be Russian in 50 years.