I know a lot of people have been having issues with "generator-settings" in server.properties not applying to flat worlds. I did a good amount of searching as was not able to find an answer.

I did come across issue #MC-182421 which stated it was fixed.
I used the settings from that issue that Bartosk posted:

{"biome":"minecraft:plains","layers":[{"block":"minecraft:diamond_block","height":1}],"structures":{"village":{}}}

I tested on 20w18a since that was the version that was claimed to work on.
It worked. But when I tested in both 1.16 and 1.16.4 neither worked. I even tested on the latest snapshot, 20w46a, and still a no go.

In the images, you will see the diamond block world worked in 20w18a, but the same value did not apply for any other version.