If a user record is set inactive (i.e., "Is Active" is unchecked) all the jobs allocated to them will be removed.
The job allocations (or JA) are deleted by a trigger. The triggered chain goes like this:
Job allocations deleted.
You cannot turn off the trigger, but you can remove the link between the resource and the user records before deactivating the user—this will give you time to reallocate critical jobs or appointments.
There is a field named "User" on the resource record. Clearing its value will remove the link between the two records. To do so, perform the following:
Using Salesforce Lightning, open the "App Launcher."
Locate and select the resource to be deactivated.
Edit the "User" field.
Clear the user name and save.
Now the user can be made inactive with their resource allocations left intact.
The job allocation status is updated to "Deleted" but the job allocation record is not actually deleted.