Note:

• Chrome and Firefox use last 3 year version rule. Currently Safari does not use such rule, probably because Safari in iOS can not be upgraded individually.
• The Grade A and Grade C defined are normative only.
  • It does not define which browser will get JavaScript (which is detected via feature check in startup module)
  • Non-JS features must work in all Grade C browsers, and JS features must work in Grade A browsers (but might work in some Grade C ones)
• Using newer JS features is not only blocked by this requirement, but also blocked by T277675: Add native support for ES2016-ES2020 or higher versions. Currently, only features up to ES7 can be used in Wikimedia code, where all Grade A browsers at least supports ES8.

However for Safari, we will drop support for version <14 completely once T367821: Discovery: Deprecation of TLS 1.2 is resolved.

https://analytics.wikimedia.org/dashboards/browsers/#all-sites-by-browser/browser-family-and-major-hierarchical-view

• Safari iOS 14: 0.38% of Safari iOS users (0.08% of all)
• Safari iOS 12: 0.5% of Safari iOS users (0.1% of all)
• Safari iOS 11, 13: <0.1% of Safari iOS users (<0.02% of all)

• Safari 14: 2% of Safari users (0.072% of all)
• Safari 13: 0.86% of Safari users (0.032% of all)
• Safari 12: 0.16% of Safari users (0.006% of all)
• Safari 11: 0.4% of Safari users (0.014% of all)

Total Safari (macOS, iOS) 11-14: 0.324% of all pageviews

Historically, we've raised the browser support requirements based on a feature we want to use that we think is worth the trade-off. GZWDer is correct that the auto-upgrade features of Chromium- and Firefox-based desktop browsers have meant that we've changed to a rolling three-(ish)-year window, and that doing this for mobile-based devices like iOS Safari and Android Chrome would be a more aggressive change.

Is there a specific feature that is being held back from production by raising iOS and desktop Safari to 15+ (i.e., every other browser including Android 5 support it)?

https://gitlab.wikimedia.org/repos/ci-tools/browserslist-config-wikimedia is where the code version of our standard lives, but let's decide first before writing patches. :-)

• Android 5, released in 2014, supports up to Chrome 95 (released in 2021) (3 years old) as well as the latest version of Firefox (2024)
• iOS 14, released in 2020, supports only Safari 14, also released in 2020.

Chrome and Firefox for Android can be updated through Play Store.

I agree that if there is no specific need for this version bump, it's probably best to wait on T367821 since that will have the same effect on the current matrix.