Apple on Thursday released its beta version of Safari 15.1 to developers. The 8th beta of macOS Monterey had it as well and was seeded this week. The software version is compatible with macOS Catalina and Big Sur.
The beta release notes are pretty much the same with version 15, with one notable exception being that it fixed a crash where the user bookmarked a YouTube page. However, 15.1 has yet to fix an issue where old tabs would reappear after the browser is reopened.

Developers can download the Safari 15.1 beta straight from the Apple Developer site, particularly the ‘More Downloads’ section as soon as they log in. To install the software, users must be running the latest version of macOS Catalina and macOS Big Sur.
Safari 15 sports several improvements, including better security, faster performance, detailed and rounder aesthetics, and more. The latest macOS Monterey update will have Safari 15 and is slated to be released this year.