I got one for getting an achivement earlier today and wanted to look up what exactly it does. The subeta.wikia says:

Open up the trunk and there'll be a message saying roughly the same thing and if I remember correctly a link to claim the csc. It'll give you a 500 csc gift card! I think the wikia just must be referring to a news post or something (in terms of the 'will soon' part)?

I must have written that years ago, sorry someone should fix that. It used to be that you didn't get to pick which Trunk you wanted at all, you got a random one. Then it used to be you bought the MCT but could pick which Trunk you got when you opened it. Then the MCT was announced to be phased out. Users loved them their trunks and didn't believe they'd be phased out, so a link to the releveant post was put in. Then the Forums got changed, so this link got broken. We didn't know this at the time but when you open them now, you get their purchase price in CSC. Yours worked like it should have.

Nowadays, there aren't any costume trunks at all.

Thanks :)

thank you for the long explanation :) so the MCT just doesn't serve its original purpose anymore, but can still be put in treasure chests or vaults etc. and be used later, right?

Yes :)