💥 Breaking Changes

Dropped support for Node.js < 18.0.0.
Older Node.js version have reached end-of-life, and certain features require new-ish JS APIs.
Pending commands no longer run when --max-processes is set and an interruption/kill signal is sent - #433, #452, #460
Before v9.0.0, pressing Ctrl+C when --max-processes is set meant that only those commands would receive it. Once these stopped, the pending commands would start, which in turn meant that another Ctrl+C press would be necessary to stop them, and so on.
Similar situation applied to combining --max-processes with --kill-others/--kill-others-on-fail.

Starting with v9.0.0, this no longer happens, as these flags and/or key presses send a signal to stop the running commands in addition to preventing new commands from running.
The concurrently and default exports are now the same - #399
It's reasonable to expect that import { concurrently } from 'concurrently' would work the same as import concurrently from 'concurrently', however this has not been the case.
The former used to be an unconfigured version of concurrently, lacking all features that you'd get from the CLI, which was seen as a "footgun". Both are now the same function.

If you'd like to access the unconfigured function, use import { createConcurrently } from 'concurrently' instead.

✨ New Features

Exponential back-off support for process restarting - #265, #462
Use --restart-after exponential. Restarts happen at 2^N seconds.
Add prefix padding via new --pad-prefix flag - #166, #417, #495
Specify teardown commands via new --teardown flag - #472, #500
Expand node:<script> commands to node --run <script> - #475
[API only] Inter-Process Communication (ipc) - #498
[API only] Make it possible to use no colors at all - #466
[API only] Add a state field to Command - #455

Fix importing concurrently in ESM package - #454, #456, #494
Make sure concurrently doesn't resolve when manually restarting commands through API - #463, #464
Fix warning coming from spawn-command when running in Node 22 - #407, #477, #479, #492
Make --raw and --hide compatible - #394, #486
Ensure prefix is written when logging without a trailing LF - #467

--help no longer shows examples due to there being too many.
Refer to https://github.com/open-cli-tools/concurrently/tree/v9.0.0/docs instead, or to the node_modules/concurrently/docs directory in your project.
Improve examples of --passthrough-arguments - #490
Replace date-fns with hand-rolled date formatting - #436, #443
This should make installing concurrently faster.

Full Changelog: v8.2.2...v9.0.0