Allies in 8th

Sorry, I'm totally behind the curve here but what are the rules for taking allies nowadays?

You can't. Allies are not a thing anymore.

Your whole army must share a key word to be battle forged.

So Aeldari allows you to have Dark/Craftworld/Harlequins in the same army - or even the same detachment - legally and without any issues. The same can be done with the Imperium keyword which allows marines/guard/knights/assassins/etc to be in the same army and detachments.

But adding tau to an Eldar army? That's a nope in battle forged lists

Ohhhh! That's what it was, many thanks!