The solution :
If you no longer remember the name of the App Service Plan, you can find it using Powershell...
$vnetObject=Get-AzVirtualNetwork -Name [virtual Network Name] -ResourceGroupName [Resource Group Name]
$snObject=get-AzVirtualNetworkSubnetConfig -Name [Name of the Subnet you want to delete] -VirtualNetwork $vnetObject
In the result, you will find the name of the ASP using: $sn.ServiceAssociationLinks.Link
Recreate that ASP, create a new functionApp with VNet Integration, using your Subnet..
open the newly created ASP, go to Network - VNet Integration and click DISCONNECT.
Give this a few minutes after which you should be able to delete the Subnet again.
Since I don't remember the names of the App Service Plan which we used, we are not able to use those solutions.
UNTIL I looked below in the article, to find a 'how to'.. Sad enough, I had come to a similar conclusion already using Powershell..
Essential = solved