Yep, turns out it is to do with animations. I had gone 10 months without the arrows before I recently got them back!

Enable Developer Options.
Enable animator duration scale to at least 0.5x. I'm comfortable with 1x.
Voila!

Or add new ones? I want to use DuckDuckGo.

OK some more investigation:
(and this is after clearing data, uninstalling, and fresh installing the apps from Google Play)

No arrows on normal Opera. No arrows on Opera Beta.

The arrows appear on Opera Mini, BUT ONLY on Extreme Savings mode. When I switch to High Savings, the arrows disappear again. No arrows when scrolling. Switch back to Extreme, the arrows come back.

What could be causing this???

I uninstalled, cleared data and reinstalled again. Still no arrows. Then I installed Opera Beta. No arrows there either.

Then I installed Opera Mini. Opera Mini has the arrows! But not normal Opera, which I prefer. Can any dev comment on this? The arrows is one of the main features that distinguish Opera from Chrome.

They're not appearing for me. I cleared the app data still no.
Then I cleared data, uninstalled and then did a new install. (From the Google play store).
Still no arrows. I'm confused.
Is there a setting in opera://flags to activate it?

One of the best features used to be the scroll arrows on the right side that enabled you to scroll all the way up or down with one tap. But they've disappeared recently. How can I reenable them? Or have they deliberately been taken out. If so, why?