Unthinkable. Look at a small /nation/ like Finland, Qtar, Kuwait, Brunei. Such a small nation incrementally builds up their rainy-day reserve, and must be very careful investing it. Investing hard-earned 血汗钱 into BTC would expose the entire nation to multiple hazards.

Reserve currency needs to be fairly stable and … with ample fresh supply. Given the technical limitation of 21 million bitcoins that you can ever dig up, BTC is unsuitable as reserve currency. Demand would far outstrip supply. Those who hold it would want to keep it forever.

As a central bank, when I need to increase my reserve I would not want to pay, say, $1b for half a bitcoin, when I could buy gold instead.

Another concern is bugs and security. Central bank governors need iron-clad guarantees that tax payers money, or decades of national savings is not subject to security flaws in an open-source software that’s influenced by a few key developers. The BTC software is controlled by some unknown SWEs, who answer to no one. From the viewpoint of central banks, Gold is not subject to manipulation, hacking, software upgrade [bug fixes or algorithm updates]. Gold is physical, tangible, can be locked up and will not disappear in thin air.

The success of ETH (or another bccy) depends on one person. If the founder goes out, it affects the entire ecosystem. This is similar to any open-source software.

Another concern is devaluation due to technology churn.

There’s also a fear that regulatory crackdown could disrupt the usability of BTC as M@E, and dampen its popularity, and indirectly affect its value.

— stablecoins .. are pegged to a hard currency. Some stablecoins could be issued by a government.
USDT is the best-known stablecoin. However, there is no guarantee provided by Tether Ltd. for any right of redemption or exchange of Tethers for real money. Worries about whether the company, accused of a lack of transparency, has enough in reserves to back the coin have been pervasive.

In contrast, fiat currencies often have “foreign reserves” maintained by national governments.

— https://www.imf.org/en/Blogs/Articles/2021/07/26/blog-cryptoassets-as-national-currency-a-step-too-far is a 2021 blogpost

— https://www.americanexpress.com/us/foreign-exchange/articles/is-global-digital-reserve-currency-on-horizon/ written after 2019, by a  regular contributor to FT, Forbes, BBC

— https://www.bloomberg.com/opinion/articles/2020-12-29/bitcoin-hits-new-high-but-cryptocurrency-s-future-is-uncertain written by a professor