This is not true, as there is... just disable the Google app.
You will need to replace the launcher with the Nova launcher in the process, and will likely replace the search box with Firefox search. But the Google app and Assistant will be totally disabled and unavailable anywhere in the Android experience. If you find a link to it, it will ask to enable which you can decline.
I found this out by Google having placed me in some experiment earlier this year where my Pixel 6 Pro went from 24h battery life down to about 3h battery life. The battery was being drained by the Google app exclusively, and so without any fix or published workaround I set about disabling the Google app.
Overall this has been a huge improvement, my battery under near identical usage is now closer to 36h... and the Assistant not being present I have since viewed as a bonus (at the time I thought it a negative). I believe the issue is probably now fixed, but why would I go back when this is a better experience with more battery, disabling the Google app was like upgrading my phone.