It's nothing like that. You can already trade currencies between players. This just means that you can buy someone's listed offer with one click instead of whispering 100 people hoping someone will respond and trade with you. Players have been asking for a feature like this for a long time and the announcement has been received extremely positively.
Plus, they're only adding it to the next temporary league (which is like a season). They said the Q&A that the intention is to keep it afterwards but if it for some reason completely fucks up the economy they still have an out.
The Steam Deck is just a Linux PC, you can absolutely mod games on it!
I’m playing modded Slay the Spire a lot on mine.