If bitcoin became illegal, I'd just travel whenever I wanted to sell it. A lot of drugs are illegal and yet they're still everywhere in most developed countries.
If mining became illegal and I was mining then I'd just sell the miner, ship it elsewhere or buy a boat to move them.
You need to differentiate between being legal tender and being banned or restricted.
Does this change based on the country? In the UK legal tender is strictly "what the courts will accept". You can pay for anything in non legal tender and some legal tender won't be accepted (such as paying for a £5000 car in pennies).
If your nation simply have yet officially accept Bitcoin and cryptocurrency, you can use it but be aware that you will not be protected by laws in your nation.
I'd reconsider this statement. If you're in a country where bitcoin is a de jure asset (eg in places like the EU where an asset has to be expressly made illegal to own) then you do have legal rights to possess the asset, trade it and receive the same contractual protections as long as you confirm yourself to their requirements (for example not doing a single transaction of more than €15000 cash without KYC).