Adaptive Notifications re-enabling itself after some updates
While this setting keeps turning itself back on after certain updates, I've finally figured out how to switch off these 'smart actions' (apparently that's their actual name - see my above comment).

As per the above, go to the Adaptive Notifications setting. Switch it on.
Go to Settings &gt; Apps &amp; Notifications &gt; Notifications.
Scroll down and hit Advanced.
Scroll down to Suggested actions and replies. Switch it off.

I'd seen this setting before, but it was greyed out. The missing step was re-enabling adaptive notifications.