Collective 2.0.0 is a major user facing overhaul for ExpressionEngine 3. It will not work with ExpressionEngine 2. However, the ExpressionEngine 2 version will remain in the downloadable zip file for a few versions so that if you need to use Collective with ExpressionEngine 2 you can do that. Collective 1.x will remain feature frozen even if Collective 2.x advances.
Collective 2.0.0 is not compatible at all with ExpressionEngine 2.
Code compatibility with ExpressionEngine 3
Collective 2.0.0 adopts the native look and feel of the ExpressionEngine 3 control panel
Added the ability to sort simple grid field items via drag and drop
Fixed a bug where submitting an update to groups might fail
Fixed a bug where not all groups might appear in the sidebar
Fixed several bugs with drag and drop sorting of various sortable items
Fixed a bug where first visiting the Collective index page would not show the variables for the first group
Fixed a bug in the update routine where the extension version might not get updated with the module version