Google adds "Lock screen" settings for the Android version of Assistant
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According to 9to5Google, in version 12.24 of the Google app, "Lock screen" settings for the Android version of Google Assistant can be found. It's part of a group under the heading of Popular Settings and it appears after Voice Match and Languages. With the settings enabled, Google Assistant can be used even when the phone is locked.

Set up Google Assistant to work even when your Android phone is locked
When your phone is locked, you will need to employ Voice Match to use Assistant hands-free to obtain personal information and to call or message your contacts. With Voice Match, the phrase "Hey Google" is required to wake up and activate Google Assistant without the use of your hands. If you decide to opt-in, you will be able to obtain "Assistant responses on lock screen" including personal info such as contacts and messages.
Things that are NOT allowed: