Workarounds

Although it's on Google to fix this issue properly, I've found a few workarounds that might get you up and running again, depending on the device you're trying to cast from. Note that I'm editing this comment frequently to refine the instructions and add new ones.

Fix casting from Android (GUI method)

deze methode heeft bij mij gewerkt

Download and install this Activity Manager app. I am not affiliated with it, but it's open source and seems to work as advertised. Note that you can download an APK directly from that page—you don't need to install F-Droid if you don't want to.
Launch the app and select "Intent launcher" from the dropdown in the upper right.
Tap the edit icon next to "Action" and paste in com.google.android.gms.cast.settings.CastSettingsCollapsingDebugAction. Leave all other fields blank. On Android 11 and below, use CastSettingsDebugAction instead of CastSettingsCollapsingDebugAction.
Tap the checkmark in the lower right.
In the settings panel that pops up, scroll down to "Connection" and enable "Bypass Device Auth".

This should fix casting from apps and partially fix the Google Home app. (It now shows me the Chromecast's status, but things like changing the name still don't work.)

Fix casting from Android (ADB method)

niet getest

Connect to your sender device (the phone or tablet you're casting from) using ADB.
Run the following:
```
 adb shell am start-activity -a com.google.android.gms.cast.settings.CastSettingsCollapsingDebugAction
```
On Android 11 and below, use CastSettingsDebugAction instead of CastSettingsCollapsingDebugAction.
In the settings panel that pops up, scroll down to "Connection" and enable "Bypass Device Auth".

As above, this should fix casting but not necessarily the Google Home app.

Fix casting from Chrome and other Chrome-derived browsers

dit lijkt niet te werken

Download this file (which is just the CA that's expired in PEM format), or copy the text into Notepad and save it as chromecast-ica-3.pem.
Launch Chrome with the command-line switch --cast-developer-certificate-path=chromecast-ica-3.pem by following the instructions for your OS here. On Windows, you might need to pass the full path to the downloaded file instead of just chromecast-ica-3.pem. (Thanks, u/yossarian_vive!)

Note that, while running with that switch, Chrome will show a notification stating "You are using an unsupported command-line flag". Seeing that is a good sign, but it doesn't necessarily mean you've specified the path properly. If casting still doesn't work, double-check the path and file name.

This adds the expired certificate as a root of trust, which bypasses expiration date checks.

Fix device setup from the Google Home Android app

Several people have reported success setting their phone's date to something before March 9th and then going through setup as normal. That method worked for me, although only initial setup worked—after setup, I couldn't change any device settings even with the adjusted date. Some people have reported that it doesn't work at all.

You can also try one of the Android fixes above, although I haven't tested if those work for setup.