How to update list item in list 2 when list item in list 1 is deleted?

Hi JeroenLaurijsen,

The problem I see with this scenario unfortunately is that using the "When an Item is Delete" step is VERY limited. The output from that trigger gives you no useful information on the deleted item. And while it DOES provide you the item's ID, using a follow-up 'Get Item' using that ID returns nothing (which makes sense... since the item is deleted. I hoped beyond hope Microsoft would somehow cache the values for us). 

So trapping the DELETE event in Flow won't help. 

An alternative, however, is to take over the delete altogether. 

1. Create a Flow where your trigger is For a selected item from the SharePoint Triggers.
2. Configure the Flow to point to your Registrations SharePoint list. 
   • This trigger now gives you full access to the item you want to delete. You'll select it in the list later).
3. Call your Flow something like "Delete Registrations."
4. In the ensuing steps, create a Flow that does essentially what your Decrement Flow does (query for the right class, get the count, and save back an incremented value).
5. Then, once you've completed those steps successfully, add a Delete Item step, and pass it the ID from the trigger item.

I didn't draw up the entire Flow, but here is the trigger and some placeholders for direction:

And because you used the For a selected item trigger and pointed it to your Registrations SharePoint list, when you navigate to your list and select a single item, when you click the FLOW menu, you will have a "Delete Registrations" option to trigger your flow.

I'd rather have the standard delete just work, but this is a viable alternative if you don't mind clicking a different DELETE button.

Let me know if this helps.