Hi Holly,
Thank you for querying in this forum.
According to your description, it seems that you encountered an warning which mentioned that the items in the list view is close to the list view threshold. And you have tried replicating the issue and creating a new view to filter, you want to prevent this situation. However, you found that the warning message in the library settings still shows the same amount of list items. And you want to know if the changes you made or items you added will have any effect until the threshold limit is met.
May I know if my understanding is right? As far as we know, if the items in a list view are less than 5000 items, it works fine, however, once the items are morn than 5000, there will appear an error message. And generally, you can store up to 30 million items or files in a list or library. Filtered views of large lists have a similar experience to other lists. However, when a list view shows more than 5000 items, you may run into a list view threshold error.
At the same time, the warning message is just a notification which reminds you there are several items in your list and it is approaching the list view limit.
In the meantime, to avoid this situation, you can edit the list view when an error occurs. For more details, you can refer to this article: Manage large lists and libraries.
If the scenario above is not consistent with yours, you can also post back and we will try our best to help you.
Best Regards,
Sukie