Ivanti EndPoint Manager (EPM) doesn't remove devices unless you have configured it to do so under Configure>Services>Inventory>Days to Keep Inventory Scans. A value of 0 (Default) means to keep all inventory records indefinitely, until manually deleted or archived. You may change this value to any number of days that you would like. 30 days is a common configuration among most of our customers.
Please know that it does not differentiate machines offline and machines that just haven't sent in an inventory scan. This setting looks at the "Last Updated by Inventory Server" value in the inventory record and compares that date to today's date. If it surpasses the amount of days that you have configured, the device is deleted from the console until the next inventory scan is sent from the client.
The following article should have more detail surrounding the options under Configure>Services>Inventory: Configuring services