Hi!
Interesting question!
Checked it in my own Innovator 12 and it seems that ReverseItemsGrid was replaced with TGVs. Aras even replaced custom views with TGVs during the last update.
Take a look at RelationshipType "Manufacturer Part Part" to see the new variant.
If you don´t want to use TGVs, you still should be able to use ReverseItemsGrid.html. Take the file from your Innovator 11 code tree and copy it to I12. I would use the customer folder for storing the file. These kind of views still works, but they are not included anymore in I12.