How to enable bulk selection for theme?

no it’s not there, but it needed to be compatible. you can see the old bulk select stuff removed here when templates were refactored:

https://github.com/discourse/discourse-simple-theme/commit/914f9695abd7938a5502355d3a17fa9c88eeb057#diff-8eab2ad083803db545ff53d060fdd09ef7881cb2708b9083dfca102438e9925d