This post is the answer to a conversation with a Senior DBA.
He asked me if it is possible to set a tablespace offline, use Rman to backup it, then remove the tablespace underlying datafiles, and, when needed restore the tablespace without performing recovery.
The idea behind is to be able to free the space used by tablespaces that we do not need to have online, but we need to keep them, usually for legal reasons, for a period of several years.
The standard solution would be to move and compress this data into historical databases, based on cheaper storage . The data transfer can be achieved using transportable tablespaces, cross-platform transportable tablespaces or data pump.
I did test two approaches of doing this: Using Transportable Tablespaces and using Offline Tablespaces.
Full details can be found here: ASM Based Tablespaces Backup With Rman for Long Term Offline Storing