John Cannon, who started Magma, pushed very hard for people to cite that paper whenever they used Magma in a paper. We eventually did something similar with SageMath for a bit, but it felt weird having everybody cite a paper with me personally as the main author, when there are nearly 1000 contributors to Sage, so we made up a reference with the author “The Sage Developers”, or something like that. As a result there is probably no MathSciNet entry tracking citations to Sagemath even though many papers cite Sagemath.