I don't know any country that has announced officially that they have approved bitcoin as a store of value but i believe members of this community use it as a store of value. if not so nobody would be holding their bitcoins after it had gained such a huge price on the market in 2017. I love to use more stable cryptocurrencies as a store of value not bitcoin.
When people say "Store" of value, they are basically talking about putting their savings somewhere it will remain safe from:
1. Hyper-Inflation
2. Confiscation
3. Theft (Phycsical or Digital)
1.) Bitcoin doesn't lose value if your home currency gets hyper-inflated. CHECK
2.) If you are crossing the border in times of difficulty, its going to be much easier to conceal/ memorize 12 letters than try to move your self-worth in Cash/ Kind.
3.) If you take standard precautions, you cannot be phished out of your money.
Physical theft is a possibility but when things come to this, you will have to defend. NOTHING is a store of value if you fail to defend your own person. This is the "Store of Value" argument. It does not need "official announcement" or "Government approval". The "stable currencies" have their value tied to fiat. They are not going to preserve your "value/ buying power"