Since it's so widely used, my question is: What is the template for this?
Since this is from arXiv, you can peruse their submission information, specifically Formats for text of submission. Here they list their preferred submission format using TeX. Quoting from the aforementioned link(s):
Submissions are automatically processed
Your (La)TeX, AMS(La)TeX, or PDFLaTeX submission will be processed automatically by our AutoTeX software.
This is a complex task, and the processing does not always lead to the desired or expected results. [...]
All TeX, LaTeX, and PDFLaTeX submissions receive the arXiv watermark, including the canonical identifier, version number, primary classification, and a link back to the correct version on the arXiv site. [...]
It is clear that the "template" output is generated in-house, and therefore not readily available. Each arXiv is available for download, but it has been constructed during the AutoTeX process.