It was a couple of months ago, when Google started rolling out the ability to activate Search or Assistant on phones by saying “Hey, Google”. It was a limited roll out initially, but the command is seeing a more widespread roll out now.
You’ll need to re-setup the voice model for this new setting to work – a notification asking the same should appear on your Android phone (if haven’t already). Manually, you can do this by heading to Settings -> Google -> Voice.
Once done, you can use both “Okay, Google” and “Hey, Google” to perform Google searches or bring up the Assistant.