Earlier quoted context omitted.
These are most likely not legally binding in the UK as consumer contracts and notices as a whole have to be fair (transparent, in good faith and intelligible) to consumers under the Consumer Rights Act 2015.
Same in Germany. There was an interesting case against WhatsApp banning a user for using a third-party app, which WhatsApp argued was against the ToS but the court ruled that the user could not have expected this to result in account termination as using third-party clients was perfectly acceptable practice for similar apps and the user was not explicitly warned this would happen. Basically you can't sneak gotchas in…
> Ironically this is unlike actual laws where ignorance is not an excuse
I think it's similar to real laws.
Unacceptable rules: sneaking in terms such as that your firstborn child will belong to me is not enforceable: nobody could expect or would agree to that. In law, similarly, you can't just upend someone's life for a minor infraction, or indeed take a child in exchange for your use of an ordinary government service. If this were to pass, a court would strike it down based on human rights or constitution, so you'd have to change/disavow those also as a country.
Acceptable rules: sneaking in terms that are fair are enforceable. Your third party client ban is an example where the rule is acceptable, but the consequences deemed disproportionate and so a warning is in order before cutting one off. This is similar to many laws, for example mildly dangerous driving gets you a fine or few days/weeks of driving ban. It takes repeat offences to get banned outright. Failure to pay taxes doesn't result in them showing up to auction your TV off, but they send a nice letter first, even if they'll eventually show up with a writ and after you're been warned that this is the next step, because indeed, we don't expect everyone to know everything.
I'm not a lawyer and can't cite relevant cases or anything, but this is my impression of law as a lay person. It's rarely perfectly fair towards everyone in all situations, and I think we should pay even more heed to "but how was I supposed to know"s whenever a situation is legitimately complex and good intent was shown (to avoid that people avoid doing good things because they don't know what legal risk it puts them in, such as what good samaritan laws are aimed to avoid), but it does try to find balances between what people can reasonably know and follow, at least in northwestern Europe which I'm most familiar with.