/seed is no longer available without cheats enabled

Can someone confirm this for multiplayer?

Traditionally the seed command isn't available to non-ops in multiplayer. They could ruin the server if they got that info.

(specifically: if they know the seed they can create a single player world with the same seed then use a 3rd party program to analyse the world and discover where every diamond cluster is in a given area. Then they can go back on the multiplayer world and mine every diamond, leaving the given area empty of diamonds.)

: (

And the problem with that is...?

That's called cheating and not something you should do on a survival multi-player server

How is knowing the seed cheating? It doesn't modify the game in any way.

Your right. It doesn't modify the game. But if you load the seed in a single player world, you can get the coordinates of where all the diamonds are. Because the world is the same always for the same seed (and same version), you can just go into that server, go to the exact same coordinates, and the diamonds would be found there. So pretty much it is cheating, if not, worse than x-ray.