I believe one of the Hive hardforks made it so only "savings" HBDs would earn interest, so presumably the behavior before that was that interest would apply to both savings and liquid HBDs, so I assume that's how Steem worked and still works.

Ah, I guess you're right. I thought I remembered seeing in Steemit's wallet that interest was paid on SBDs in savings, but not on the first SBD line, but I don't see that now, so could be wrong. It looks like they've removed all references to SBD interest now.

I do see it like that in upvu's wallet:

but that could be because they cloned the code from ecency, which had migrated to Hive.

So yeah, all bets are off... Thank you for catching that. I'll have to rethink things with that change in mind.