Upgrade Behavior Options of Unlocked Packages :

 Options when installing the unlocked packages.

1.Mixed Mode (Default behavior) :

Specifies that some removed components are deleted, and other components are marked deprecated.

2.Deprecate Only :

DeprecateOnly specifies that all removed components must be marked deprecated. 

The removed metadata exists in the target org after package upgrade, but is shown in the UI as deprecated from the package. 

This option is useful when migrating metadata from one package to another.

3.Delete :

Delete specifies to delete all removed components, except for custom objects

and custom fields, that don't have dependencies.

ex :

sfdx force:package:beta:install --package 04t... -t DeprecateOnly