As of 13.2 the portfolio module was completely redesigned, not much knowledge can be taken over.
It really depends on your business use case and how your organisation uses portfolios. I've seen them used as project lists to financial spend control instruments. I.e. shutting a portfolio down might involve updating other objects.
Typically you'd just remove any sync schedules, then delete if no longer required.
However It's certainly possible to hide these "inactive" portfolios from view too.