According to a report published by ZDNet today, Microsoft has just made a whole pile of its IP available in a "royalty-free and unrestricted license" by joining the Open Invention Network (OIN), a shared defensive patent pool launched to protect Linux. By pledging these patents to the group, Android OEM members of the pool should have that same royalty-free access to the relevant patents, which cover Linux and Android-related technologies.
ZDNet cites an announcement from Microsoft on the subject that we aren't able to find, but claims the details were further confirmed in a later interview with OIN's CEO, Keith Bergelt, who is reported as saying:
This is everything Microsoft has, and it covers everything related to older open-source technologies such as Android, the Linux kernel, and OpenStack; newer technologies such as LF Energy and HyperLedger, and their predecessor and successor versions.
This is alleged to bring to the pool 60,000 Microsoft patents, which earned the company as much as $3.4 billion a year in Android patents licensing (based on numbers from 2014). At one point back in 2011 during peak Balmer-era, Microsoft was aggressively pursuing licenses for its patents against over half of all Android OEMs. Times (and CEOs) change.
Microsoft is now listed among the OIN licensees. Other members and licensees include Google, LG Electronics, ASUS, and Sony.
Previously some OEMs entered into agreements with Microsoft individually, exchanging those licensing fees for pre-loading Microsoft apps. There's no telling on how existing patent licensing deals could be affected, but presumably, Android OEMs which join the OIN will be in a better position going forward.