If a sovereign government declared bitcoin illegal, it becomes illegal. There is nothing unique about any asset to stop this because the rules are all made up and can change anytime.
Crypto is arguably among the worst assets for that threat model though.
When they called in the physical gold, people could squirrel away their $10 and $20 coins in wallsafes and shoeboxes to trade with trusted partners later. Declaring the coins illegal didn't cause them to disappear from people's hands directly.
Crypto doesn't have that physical anchor. If a major world government declared it illegal tomorrow, how would you, as a resident of that country, access the value in your favourite blockchain wallet?
Law enforcement could monitor the remnants of the network and trying to trigger transactions is obvious probable cause. That assumes the remnants still function-- they aren't being blocked by DPI/filtering, or just the functional destabilization caused by a large number of participants being unplugged from the network.
At best, you'd have to go off-ledger and trade talismans (handing over a document with the private keys for an existing wallet like a bearer instrument and hoping they didn't run off multiple copies, or just trying to keep records of transactions and balances elsewhere and hoping to resynch eventually)
This is also why the paper dollar is so important to its reserve-currency status. An enterprising bank in the developing world might say "we offer USD-denominated accounts, all digital, it's 20xx" for customers who don't trust the local currency, but those are a lot easier for the local government to deactivate than a coffee can full of greenbacks buried in their backyard.
The context of my comment was Bitcoin as an alternative to traditional reserves. I’m saying that in a domain as lawless as international law, gold bars make sense in a way that bitcoin does not.
Split the user base along geopolitical axes and see how long the consensus algorithm lasts.
Gold bars don’t have this problem.