I know there are apps to automatically tag products, however, I've just tried one and it doesn't seem to untag them!
Are there any apps that automate tagging AND untagging?
I can recommend checking out Arigato Automation. With this app, you can create an automated task to tag products/customers/orders when specific conditions are met. You can also remove the tag after a custom delay(hours/days/etc). Here is an example workflow from our automation library that can help to quickly get started: Tag new customers with the "New member" tag and remove it after 6 months
You can also reach out to the support team here if you need a hand with the implementation.
Unfortunately a time delay will not work.
I've migrated a store to shopify and there are quite a few products that are out of stock and I'm using manual collections which means I can't utilize the automated collection trick to hide out of stock products from a collection.
I'm waiting to leverage an automated tag that will add and remove itself depending on stock level. This way I can filter the items out of a collection and still have working pagination.