I know that IP ranges can be returned to RIRs and reassigned, but what about AS numbers?
AS numbers can be reassigned; quoting RIPE Autonomous System (AS) Number Assignment Policies and Procedures, Section 1.8:
1.8 Returning an AS Number
If an organisation has an AS Number that is no longer in use, it can be returned to the public pool of AS Numbers by sending a message to email@example.com. It can then be reassigned to another Autonomous System by the RIPE NCC.
I can't find a similar public statement from ARIN, but I know someone personally who received a reassigned ASN from ARIN.