My 20 EUR Claude (individual) had the training on by default.

Yes, individual indeed (used to different but changed about a year ago, see [0] among others].

For the Team and Enterprise plans de default is "not sharing prompts for training" (which I mistakenly referred to as opted out of sharing prompts for training.) [1]

[0] https://news.ycombinator.com/item?id=45076274

[1] https://claude.com/pricing#team-&-enterprise