It is not under Export any longer. From the 3D window, File > Save As and select the file type of BIMx. You'll probably want to check the box there for 'Start BIMx after save' to get the same behavior as 16 and earlier.
Click the Options button to review before clicking Save....
One of the forum moderators
AC 27 USA and earlier • macOS Ventura 13.6.6, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB