Let's say an internet service provider (ISP) bought 1024 ip addresses from IANA and after 1 year that ISP business fails and it returned those ip addresses back to IANA. Now there must be a hole of free ip addresses that is created in the ip address pie. How IANA handles these free holes which eventually accumulate overtime.
I dont know how IPv6 are assigned but I'm familiar with IPv4. First ISP's reserve IP classes not specific IP addresses so for example, they reserve IP class A IP
120.255.255.255/8Which gives them so many hosts IP range that the ISP can internally allocate to it's users, So if the ISP went out of business only one free IP class will be free for others to use.Also, IANA allocate IP blocks to Regional Internet Registries(RIRs) around the world which are 5 (Africa, Europe, ...) continents. Those RIR will then allocate smaller blocks to the ISP which in turn will allocate single IP address to its customers, so the whole system is hierarchical in nature.