What's Changed

This release prepares jQuery 3.x apps better for a future migration to jQuery 4.x. It also drops support for jQuery 4.x - once you do update to jQuery 4.x, you should load jQuery Migrate 4.x.

Core

• Fill in & warn against $.fn.{push,sort,splice} by @mgol in #529
• Fill in & warn against jQuery.proxy by @mgol in #530
• Fix the selector module, make CI green by @mgol in #552
• Remove support for jQuery 4.x by @mgol in #554
• Patch push, sort & splice in jQuery >=3.7.0 by @mgol in #568
• Warn against jQuery.now & jQuery.camelCase by @mgol in #600

Ajax

• Fill in & warn against automatic JSON-to-JSONP promotion by @mgol in #531

Attributes

• Warn & restore boolean attribute & false setter treatment from 3.x by @mgol in #540

Data

• Patch camelCase behavior of $.fn.data, warn about Object.prototype by @mgol in #559
• Patch $._data & $._removeData as well, add more tests by @mgol in #570

Deferred

• Fill in & warn jQuery.Deferred.getStackHook by @mgol in #528
• Don't warn on setting getStackHook to the getErrorHook value by @mgol in #578

Event

• Patch jQuery.event.special's prototype by @mgol in #572

Selector

• Patch & warn against legacy selector pseudos by @mgol in #546

Others

• Docs: Add an entry for legacy-self-closed-tags in warnings.md by @mgol in #526
• Docs: Tweak README.md & warnings.md - update removal info by @mgol in #596
• Docs: Indicate which deprecated APIs are removed in 4.0; tweak a comment by @mgol in #606
• Tests: Migrate test runner to jquery-test-runner by @timmywil in #560
• Build: Test more browsers and jquery versions in pull requests by @mgol in #551

Full Changelog: 3.5.2...3.6.0