Astrophysics papers often rely on software which may or may not be available, and URLs are often used as proxy citations for software and data. We extracted all URLs from two journals' 2015 research articles, removed those from certain long-term reliable domains, and tested the remainder to determine what percentage of these URLs were accessible in October 2018.
@article{arxiv.1903.06821,
title = {Availability of Hyperlinked Resources in Astrophysics Papers},
author = {P. Wesley Ryan and Alice Allen and Peter Teuben},
journal= {arXiv preprint arXiv:1903.06821},
year = {2019}
}