Ok I got you. So you need to approach the issue from another side, because of that rule that when Fieldabc__c this is not counted as object update.

1. Make a new date time field called LastDateModifiedSpecial__c

2.Make a process that is triggered when the object is created or edited. in this process use the formula ISCHANGED(field) in a decision element to see if your special field is updated or not.

3. If it is not updated just copy the standard field LastUpdated date time in your new custom field. If the special field is updated do not change the value in your special field.
4. Then use this specialLastDate field to calculate the InactiveTime__c in a formula field of the object.
5. Then you would need to trigger and update on all records so that you trigger this new procces and so your ne field is populated for old records

Please tell me if you got my idea and if this solved your issue.

Ok I got you. So you need to approach the issue from another side, because of that rule that when Fieldabc__c this is not counted as object update.

1. Make a new date time field called LastDateModifiedSpecial__c

2. Make a process that is triggered when the object is created or edited. in this process use the formula ISCHANGED(field) in a decision element to see if your special field is updated or not.

3. If it is not updated just copy the standard field LastUpdated date time in your new custom field. If the special field is updated do not change the value in your special field.

4. Then use this specialLastDate field to calculate the InactiveTime__c in a formula field of the object.

5. Then you would need to trigger and update on all records so that you trigger this new procces and so your ne field is populated for old records (you can do that using a scheduled flow or a dataloader import updating a dummy check box field on the object to trigger the update and the initial population of your new LastDateModifiedSpecial__c)

Please tell me if you got my idea and if this solved your issue.