How to turn off auto suggestion popups in Google Keep?
There are some workable workarounds buried in this forum thread.

The simple way (which makes writing lists take slightly longer):


Initially, create the list as a normal note with one line per item (not a checkbox list) 


OR if it's already created as a checkbox list, press 'Hide checkboxes' under the three vertical dots in the top right. Unfortunately this causes Keep to forget which items were ticked.

Finish writing or editing the list
Press the '+' in the bottom left corner, then press 'Tick boxes' to turn it back into a check list


The more involved way (which in theory, you can do once then forget about)


Install a url filter like an adblocker and add the URL for Google's "show some popups of things I didn't type" service to the list of banned addresses. I've not tried this, so I can't guarantee it works - in particular, I don't know if it requires rooting the device or not (please comment if you have tried something like this). Two suggestions from that post:


The app uBlock, then going to "Settings -> My Filters" and adding https://taskassist-pa.clients6.google.com/v2/taskassist:suggest*
AdBlock, then going to "Customize > Manually edit your filters" and adding /v2/taskassist:suggest$domain=taskassist-pa.clients6.google.com 



Don't forget to also go to "Help &amp; feedback" in the app and tell Google that you want a normal option to turn off auto suggestions.