Platform: Apple TV HD / Apple TV 4K
Release date recorded by Apple, not the document revision date.
Apple source ↗Apple dates tvOS 15 to September 20, 2021, for Apple TV HD and Apple TV 4K. Juli Clover's launch report described Shared with You for recommendations received in Messages and For All of You for group suggestions. The shared living room gained collective recommendations without discarding personal accounts.

Apple · tvOS 15 security content · Juli Clover · MacRumors · tvOS 15 · 2021-09-20
Clover also documented spatial audio with AirPods Pro or AirPods Max and two HomePod mini speakers used with Apple TV 4K. These are specific setups, not a promise of spatial audio on every headset or identical capabilities on every speaker. Multiple HomeKit cameras could be viewed together.
Juli Clover · MacRumors · tvOS 15 · 2021-09-20
SharePlay is not treated as available in 15.0: in September Clover still expected it later. Apple's archive dates 15.1 to October 25; the October 28 developer notice confirms SharePlay available in tvOS 15.1 with iOS and iPadOS 15.1. The notice date does not replace the system release date.
Juli Clover · MacRumors · tvOS 15 · 2021-09-20 · Apple · Security updates archive · 2020-2021 · Apple Developer · SharePlay · 2021-10-28
The table gathers Apple milestones through 15.6, keeping tvOS separate from other Apple TV software lines. It is a sourced selection, not every change or a guarantee of current support. Security-document publication and revision dates are not used as software release dates.
Apple · Security updates archive · 2020-2021 · Apple · Security updates archive · 2022-2023
Selected releases, not a complete history. Dates follow Apple's release records, not the latest page edit. No builds are included.
ISO date: year-month-day.
Platform: Apple TV HD / Apple TV 4K
Release date recorded by Apple, not the document revision date.
Apple source ↗Platform: Apple TV HD / Apple TV 4K
Software update released according to Apple archive.
Apple source ↗Platform: Apple TV HD / Apple TV 4K
Software update released according to Apple archive.
Apple source ↗Platform: Apple TV HD / Apple TV 4K
Software update released according to Apple archive.
Apple source ↗Platform: Apple TV HD / Apple TV 4K
Software update released according to Apple archive.
Apple source ↗Platform: Apple TV HD / Apple TV 4K
Software update released according to Apple archive.
Apple source ↗Platform: Apple TV HD / Apple TV 4K
Software update released according to Apple archive.
Apple source ↗Platform: Apple TV HD / Apple TV 4K (1st and 2nd generation)
Software update released according to Apple archive.
Apple source ↗Platform: Apple TV HD / Apple TV 4K (1st and 2nd generation)
Software update released according to Apple archive.
Apple source ↗Platform: Apple TV HD / Apple TV 4K (1st and 2nd generation)
Software update released according to Apple archive.
Apple source ↗