I was pretty sure that there is something and a lot of searching finally led me to the “Unfair Commercial Practices Directive” from 2005. There also is a guidance to that directive from 2021 that is found here: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A52021XC1229(05)
Section 4.2.7 (dark patterns) is what is interesting for this topic. In the paragraph at the end of the section it includes the sentence:
unsubscribing from a service should be as easy as subscribing to the service
So it appears that the EU intended it that way but because it’s only a directive, implementation differs by country. I also didn’t see anything about being able to cancel in the same way you subscribed(e.g. that they can’t force you to call or send a letter if you subscribed online), but afaik german law has a ruling like this.
Edit: I took so long to write this and find the links that I forgot the german law was the reason for the comment I answered…
You could probably use your phone. I haven’t tried it but I’m pretty sure it’s possible.
And you don’t need a bootable USB for every OS you have. You only need one and can create whatever you need on that live system. In case of the steam deck you could also have an SD card with a different OS and create your SteamOS stick there.