You certainly have the ability to delete your custom contact properties in HubSpot; sometimes their use runs its course and you no longer need that property. However, when deleting custom contact properties, you'll want to consider whether or not those properties are being used in forms or to enroll contacts into lists or active workflows.
When you delete a property, HubSpot will warn you if this property is being utilized in other assets of HubSpot.
In the above example, you can see the property about to be deleted is being used in a list, workflow, and form. The prompts tell you how you can adjust your assets before you delete your property. If you're okay with how deleting this property will affect these assets, click Delete property to go ahead with deleting the property from HubSpot.
If you've deleted a contact property in error, you can restore it by creating a new property with the same internal name and property type.