PSL is used to determine the level that a unique domain is registered at. This restricts cookies and privileges to that domain. It’s just a simple list because both .com and .co.uk are valid suffixes. Ios14 is using this list to prevent apps from tracking you across sites by limiting the data that can be stored per site. If you can get your domain recognized as a suffix as mysite.com then you can split information between all higher level domains. client1.mysite.com and client2.mysite.com. This allows you to store as much information as you want.
The PSL has always been a giant hack and totally unmaintainable in the long term. It's only a matter of time before someone mistakenly relying on it for security purposes gets owned by a rogue PR. Also, as mentioned in some of these issues, browsers don't even update it on any sort of guaranteed schedule.
The fun thing is you can s/PSL/DNS/g and the statement still holds. Same for BGP
BGP and DNS survive due to relative obscurity, despite everyone using it in some way. Web devs do not give a shit about BGP (sadly.)