The dictation UI for devices that support new Google Assistant — the Pixel 4, 4 XL, and 4a — is advertised with a launch prompt encouraging users to tap on the microphone icon. They'll then see further voice prompts to "Close" the UI, "Delete" words, or "Send it" to send the message. Pixel owners were able to take advantage of a similar though limited Assistant voice typing UI sans Gboard for months.
These dictation controls were introduced in a demonstration of various and continuous Google Assistant commands during Google I/O 2019, though the UI itself was nothing like the one we're seeing today.
If you don't have a Pixel, but have been missing the ability to undo Google's autocorrections with the tap of a backspace, it seems as though that ability is coming back. We were able to grab the above screenshot from beta v9.7.03. XDA-Developers reports that v9.7 is headed into stable release right now.
On the opposite side of anti-completionism, smart completions (or, as we've dubbed it, Gboard's version of Smart Compose) also has a toggle here after having hung around since beta v9.5. Again, it appears as though this may have slipped through to some people on the stable v9.7 release.
For what it's worth, on our beta app, the smart completions toggle appeared after the undo autocorrections toggle disappeared, so take that for what you will.