If you're asking for a clause in the ToS or the EULA then no you won't find one.
But generally the practice you're asking about falls under consumer-business goodwill. That the expectation, quality, and cost of services rendered wont suddenly change.
It's the same reason players don't expect them to suddenly start charging for any other number of game interactions that we currently have access to even though they technically could, according to the ToS and EULA nothing really stops them, they have the option should they wish. However we dont expect them to start charging for raids resets (some free to play games actually do charge for this) or various other game features, because we pay a recurring subscription. It's one of the draws of the game itself.
Its part of the subscription based MMO model.
And it doesn't always work, you know why?
Because people speak up about it or take their business elsewhere.
If everyone just shrugged their shoulders anytime it happened it would be far more commonplace. Its why we give feedback, surely such a concept isn't that foreign is it?
In all honesty it doesn't affect me, I have the items, I was there for the events.
But judging by the thread it effects other people, and as a consumer I can speak up for and agree with them as well.
When I used the quote for the statement of the argument I don't mean myself personally, but it is an illustration of what the situation actually is. And it's not about whether or not you take something with you when the servers shut down or what content is no longer in the game but the user experience and consumer/business interaction.
SE as a company wont know what is and isn't okay if people don't give feedback on the matter, which is... exactly what I'm doing. I'm not sure how you haven't picked up this point across 3 posts, it hasn't exactly been subtle.
But if you wish to sit there and say "Well it's fine the way it is!" then by all means continue to do so, that's your right as a consumer, just as it's mine to disagree with the practice.