Let's break it down...
"We need a 'trip mode' for social media sites that reduces our contact list and history to a minimal subset of what the site normally offers."
If you don't want things in public, don't put them on social media.
"Not only would such a feature protect people forced to give their passwords at the border, but it would mitigate the many additional threats to privacy they face when they use their social media accounts away from home."
No it wouldn't. The oligarchs who want the data will just get it via other means. "Giving passwords at the border" is a convenience for them, but not the only way to get the data. And what are these "additional threats to privacy"? That's just meaningless add-on to the sentence. You created the threat to your privacy when you posted the information in public.
"Both Facebook and Google make lofty claims about user safety, but they've done little to show they take the darkening political climate around the world seriously."
Facebook and Google never have and never will care as much about your privacy as you do. They MAKE MONEY off of mining your information! And another meaningless sentence add-on... "darkening political climate"... huh? When did governments stop wanting information on travelers, ever?
"A 'trip mode' would be a chance for them to demonstrate their commitment to user safety beyond press releases and anodyne letters of support."
And it would be a false sense of security. All it takes is a subpoena or a claim that you're a "terrorist" to get any social media company to quite-willingly hand over whatever law enforcement wants, without you even knowing about it.
"What's required is a small amount of engineering, a good marketing effort, and the conviction that any company that makes its fortune hoarding user data has a moral responsibility to protect its users."
Or just stop feeding them user data.
"To work effectively, a trip mode feature would need to be easy to turn on, configurable (so you can choose how long you want the protection turned on for) and irrevocable for an amount of time chosen by the user once it's set. There's no sense in having a 'trip mode' if the person demanding your password can simply switch it off, or coerce you into switching it off."
They can switch it off whenever they like... it's called a subpoena. You're fixing the wrong problem putting a "mode" in the user front-end. What's needed is encryption on the back end and even the company "hoarding" the data you willingly gave them NOT being able to read it at all, which... obviously isn't their business model...
The key thought here is, you do NOT need social media. No one NEEDS social media. Whatever you GIVE WILLINGLY to a company about yourself is easily accessed by anyone who can even hint that you are some sort of "threat" to anyone in society. No "mode" will fix that. Just STOP providing the information if you don't want it seen by everyone.