OK, I’m going to be honest and say that the tone and approach of this article is pretty bad. It flips from a sensible “we should have a discussion about trail rules”, to “this is already the rule and you should already know”. And throws in a patronising “for new riders or those who haven’t been out in a while, they may not be aware”
There is no “crossed skis” equivalent in mountain biking. It’s not an “unwritten rule”. It has none of the acceptance that’s needed to make that the case, if you want it to be, we’re pretty much starting from scratch.
Seriously, if you asked 100 people, how many do you think would give this answer? (I don’t mean like I would answer with common sense, “maybe something’s wrong”, I mean “upside down bikes on the trail is the signal for an accident”) Try googling “upside down bike accident” and “crossed skis accident”, bit different. (literally the only hits on my first 2 pages for the bike one are this article. OTOH there’s a hell of a lot saying “never turn your bike upside down”)
Upside down bikes like in the photo are a good way to block a trail/warn people, yes. And also suggests possible mechanicals- if I see someone with an upside down bike at trailside I’ll ask if they need help. And it’s common sense that if someone’s made an effort to block a trail you should react and think about why… If we’re going to have a rule, it’s a good one.
If you want to create and popularise a rule or rather set of rules, work to publicise it and get it accepted enough so that it’s actually a useful standard, fill your boots, it’s a good idea. It’d be a useful part of a whole brief guide to “what to do if there’s an accident”.
You don’t do that by just pretending it’s already the case and that people should already know. Good idea; wrong message, wrong delivery. And that’s a shame.