Then there is Tuvalu's TLD .tv that generates around 8% of the country's total government revenue. Due to rising sea levels, the pacific atoll nation may not exist much longer.
There is also the double barrelled but still active oz.au domain hierarchy - so good they named it twice! Back in the early Australian Internet days we admin'ed pegasus.oz.au for one of if not the first major national internet access networks in Australia.
Not a TLD such as .io, but In terms of Internet domain diasappearances, there is the legend that is fuckedgoogle.com, an anonymous economist's massive click-fraud expose of Google's business model and practices. The true reason for it's disappearance is made of legends and conspiriacies. See What Happened to fuckedgoogle.com for a tasty intro to the aftermath.
"A still-underappreciated truism of our moment is that there is great solidarity to be found in refusing a technology."