To our knowledge, Bitcoin has not been declared illegal in most countries.
However, it is still possible to regulate the use of Bitcoin as well as to regulate other financial instruments. Just like the dollar, Bitcoin can be used for various purposes, some of which may be considered legal or illegal depending on the laws of a country. Related to that, Bitcoin is no different from other devices or resources that must follow different rules in each country. Bitcoin can also be complicated by strict regulations, making it difficult to determine how many percents of users will keep using this technology. The government that prohibits Bitcoin will prevent its business and domestic market from growing, thereby shifting the chances of innovation to other countries. The challenge for policymakers, as it always does, is to develop efficient solutions without hindering the growth of newly emerging businesses and markets.