Considering that the related filter still exists, if I delete a label in Gmail where will the mail be?
If you delete a label that was being applied in a filter, the specific action of applying that label will simply disappear from the filter. So if the filter only applied that label and the label is deleted, the filter will still exist but do nothing.
As filters can have more than one action, the other actions, if any, will still be executed when emails with matching conditions arrive.
So the email will still arrive and end up in the Inbox, unless one or more rules move it away from the Inbox.
If the original rule applied the specific label and then archived the email, the email will still end up in the "All Mail" folder after the specific label was deleted.
Shortly: Deletion of a label is NOT affecting the existing emails. Not at all, no way.
More broadly: "where will the mail be?" - Still on the same heap of all emails: There are no folders in gMail, just views on grouped emails; sub-heaps defined by labels.
- where files are nested under/in the folders (below/inside them, in exactly ONE nesting dir), so when the directory node is deleted, all the file underneath files are gone too;
the email/labels behave differently.
Notice, that the relation is opposite: Email is the node, labels are coloring it. "Labeled" is just a property of an email: There can be many labels on an email. So any label can be removed freely from any email. Even the label:inbox is just a common label. ;-) However, these two types of objects are independent, from the prerspective of the structure, the deletion: Delete one, the other remains untouched.
Also you should just go, create such case (label+filter), and then delete the filter: gMail has the alerting pop-up for years, explaining, that nothing will happen to the marked emails. Very basic misunderstanding in such question. :-/
- Filters often do just a single action: Labeling. Once any label is removed, the whole list of filters should be checked (manually Ctrl+F) for missing "targets". So I can just recommend to look up for the objective label in the filters, and manipulate the filters in advance, before the label is deleted. (I keep my gMail open in two tabs permanently: Inbox, and settings/filers.)
- The filter actions, i.e. the labeling, is applied only on the just coming mail: There is exactly one fire-up of the filtering, when all the filters are tested for the single new email / applied on it. So these single-moment new-mail events are not comparable to any changes of the filters. You can do whatever you want with your filters: You can submit the changes, and new email arrive meanwhile, but these two fires never collide. The filter is in version 1, or version 2, for the new email; nothing between. So the situation is crystal-clear all the time.