Is the definition of sexual misconduct now obsolete?

Since the norms of society has changed greatly since the Buddha's time, is the definition of sexual misconduct now obsolete?

Not entirely obsolete.

My personal experience is that my wife and I started dating each other when we were teenagers. At that age we were each "under our parents' protection" (i.e. living with our parents), and our parents had (it seemed to us) control over that: e.g. who we were allowed to "go out" with (or even go on holiday with), where, when, to do what, using whose car, whose driving, what time home, and all that.

Sometimes they'd allow a date, sometimes they wouldn't. And our families (parents) knew each other.

Sorry if this is too personal an answer; it's my experience.

This tradition would have provided little scope for sex outside of marriage

I guess you're making some assumptions based on some view of historical society.

Even so I think that norms you quoted are still normal at least to some extent: parents do (or should) take an interest in who their children are dating -- whether they seem to be "suitable" partners etc.

Maybe not "arranged marriages", in the sense of marrying someone who you've never met before, but even so.

I'm not sure how much this advice was adapted to the modern world (where potential partners evaluate each other, and the decision isn't only made by parents and match-makers), but that suggests that some getting-to-know-each-other before marriage maybe wasn't entirely wrong even when the suttas were written.

I do recall a reference in a sutta to partners "being given" to each other -- FWIW that's a conceit of contemporary traditional (Christian/Western) marriage too (e.g. the bride being "given away").

And I think, don't you, that it probably is still widely seen as dubious -- immoral, disreputable, scandalous, creepy, even illegal -- for someone (especially an adult) to get or to seek to become sexually involved with those who are under the protection of their parents ... and ditto other people's wives or fiancées, etc.

From another more modern translation (Kelly, Sawyer, Yareham) of DN31:

"And, the mother and father so respected reciprocate with compassion in five ways: by restraining you from wrongdoing, guiding you towards good actions, training you in a profession, supporting the choice of a suitable spouse, and in due time, handing over the inheritance.

In the Piya Tan commentary, it says:

Patirupena darena samyojenti, lit “they have him bound to a suitable woman,” which is said in reference to Indian society in the Buddha’s time. In contemporary terms, this has to be contextualized to “they let their son or daughter find a suitable spouse.”

Also, we can assume that adult men and women who no longer stay under the care of their parents and become independent, would no longer fall under the category of being protected by their parents. And vice versa.

So, while the definition of sexual misconduct has not become obsolete, it has become "updated", so to speak.

Take it back to first principles. Particularly the first precept: not to cause harm.

If I engage in sexual activity with someone who is at the time too young to fully consent, I set in place conditions for future suffering.

If I engage in sexual activity with someone who has committed to sexual fidelity with another, I set in place conditions for future suffering for their partner.

If I engage in sexual activity with someone over whom I have excessive power (e.g. as a school teacher, meditation teacher, etc) I set in place conditions for their future suffering when they realise I am not perfect.

In relation to the quotes you mention. Engaging in sexual activity with someone who's parents deem themselves to be protecting. This is difficult. I could deem myself to be protecting my (fictional) 45 year old daughter. But do I have the right to do so? When is that protection actually in the best interest of the child? In modern societies we usually accept that once someone is an adult (over 18) they have the right to decide who they have sexual relations with, and given no other mechanisms for deciding, that would seem a reasonable approach to follow. Although, if I were to engage in sexual activity with someone whose parents would seriously disagree yet was over the age of consent, I would be sure to discuss with them the potential consequences for themselves (e.g damaged parental relationship) before doing so.

Thus, what matters is the first precept: not doing harm.