Well, in the EU you must declare everything over 10.000€.
You could argue you're not bringing anything physical. Otherwise hiding it from customs would be illegal.
Bitcoin isn't in your wallet, Bitcoin is on the blockchain. You may have access to it, but that's kinda like having access to your bank account when you're abroad.
You're probably right, although it could be argued that e.g. a physical coin with a prepaid balance and a seed phrase (or qr code) hidden inside of it would potentially fall under declarable goods, as whoever owns it would have access to those bitcoins. It wouldn't be quite the same as having internet bank account. So potentially the method of storing bitcoins could matter for crossing a border.
I suspect in some countries that could still be a grey area and subject to customs officers' interpretation.