There is now an Agenda to designate an assimilation target instead. You can take the Conquest tradition to speed it up if you want to add even more biological and technological distinctiveness to your own.

So its a wanted change ? , is there a way to get an older version of the mod ,waiting 20 years for one attack is not rly exciting

No, you can take the Conquest tradition tree though.

then i guess it's bugged because i use an old safe where the Conquest tradition tree is already completed ,and i cant declare war without the concil agenda

It's not bugged. The Conquest tradition will reduce the time it takes to run the agenda in half.

This will be in the next patch.

A bit unrelated to this issue but ive noticed a bug with the new version 3.10 and the mod. Since the councilors were reworked the head of science and military arent recognized by the game and so the player gets a penalty of 25% to both. Edit: thx for fixing it.