Macs bought in China can no longer display the 🇹🇼 Taiwan Flag Emoji, no matter which region is set in System Preferences

To go along with the other answers and the info on the wiki page an analogy might help:

Imagine there was another civil war in the United States, but instead of states trying to separate, this one was about competing forms of government. Let's say it's the United States of America against the revolutionary People's States of America. The USA and the PSA both claim to be the "real America" and they spend years fighting a bloody civil war for control. Eventually the PSA forces are able to overwhelm the USA government forces and take the mainland, with the government of the USA and many USA refugees (along with a bunch of gold and museum pieces from the Smithsonian) fleeing to Hawaii which is too hard for the PSA to take.

At this point both sides are in a stalemate and both claim to be the legitimate American government. Schoolchildren in the USA learn they'll one day take back the mainland, and children in the PSA learn that Hawaii is rightfully theirs. The PSA controls the most people and territory, including the capital and the original 13 colonies, but the USA is the "original" republic and is still recognized by the rest of the world as the legitimate government. The USA still holds their seat at the UN including their seat on the UN security council, and nations around the world maintain relations/embassies with the USA and shun the PSA. When people talk about America they mean the USA and when they want to talk about the PSA they call it "Red America" or "mainland America."

As this stalemate persists though things begin to change. The PSA is a lot larger than the USA (which is really just Hawaii) and the rest of the world is eager to normalize relations and do business with them. It's an absolute non-starter for the PSA to recognize the USA as an independent nation so countries generally agree to a system where only one of the USA or PSA can be considered America. Most of the world recognizes the PSA as the legitimate government of America (including the PSA taking over America's seat at the UN from the USA) and they move their embassies there. Technically most countries don't recognize the USA at this point but everyone still has de facto relations with them. The USA and the PSA themselves come to a sort of understanding where they each claim to be the official government (and they each claim each other's territory) and a more permanent solution is left for future generations. Critically, they each agree that there is only one America.

Interestingly, in Hawaii, a bit of a cultural shift takes place over the decades. Though their government is still officially the United States of America, and they technically claim all of the mainland, people aren't necessarily thinking of themselves as Americans anymore. The new generation is growing up with more of a Hawaiian identity and they recognize that Hawaii has its own unique history separate from the mainland, including their own indigenous language and the fact that the PSA never controlled Hawaii at any point in history. Many people consider themselves as culturally Hawaiian or both Hawaiian and American, and fewer people see their national identity as being American.

In the rest of the world people are also forgetting that Hawaii is technically the "United States of America." The de facto country is routinely referred to as Hawaii (even in news reports), its citizens are referred to as Hawaiians, and even its flag (the old 'Stars and Stripes') is often called the "Hawaiian flag" instead of the "American flag." The PSA is sensitive about this, though, and they use their clout to force international organizations from the Olympics to Esports to never refer to Hawaii as "Hawaii" but instead to use some euphemism like "American Honolulu." They even keep computers in the mainland from being able to display the Hawaiian flag emoji (which is technically the USA flag emoji).

Now, in this situation, why doesn't Hawaii just say, "hey everyone, we're not the USA anymore, we're actually just the nation of Hawaii and we don't make any claim to the mainland"? Partly it's because of complex issues of identity and history, but mostly it's because the PSA has a bunch of missiles pointed at Hawaii and has promised to invade them if they make any move towards official independence. So, the whole world kind of just accepts the status quo and hopes that the issue will be settled peacefully in the future without triggering a massive war.

Obviously this analogy doesn't convey the complexities of the real life situation between Taiwan and China (i.e. between the Republic of China and the People's Republic of China) but maybe it offers a fresh way to look at it without preconceptions about what exactly China and Taiwan are. To sum up: the "Republic of China" formed in the early 20th century after the fall of the last emperor, there was a communist revolution which claimed to rule China as the "People's Republic of China", a civil war was fought between the ROC and the PRC, the ROC government fled to the island of Taiwan, both initially claimed to be the real China, eventually Taiwan formed its own national identity separate from being just the ROC, but if Taiwan declares themselves to be independent (i.e. to give up their claim to be China) then the PRC has promised to go to war over it.

Two important points not present in the analogy I made:

1. Taiwan is a lot closer to mainland China than Hawaii is to the mainland US. Taiwan is very much within China's geopolitical sphere and they are very vulnerable to an attack from the mainland, including short range missiles (of which China has many). At the same time though, Taiwan has a modern military and a very difficult geography for attackers to deal with. Despite China's massive size any invasion would be very costly and it would be controversial to argue that it would be a sure victory.
2. Ownership of Taiwan is very important to China not just strategically (China's coast is basically blocked in by a bunch of islands including Taiwan) but culturally and emotionally as well. It's hard to overstate the importance of this. I'm starting to get a bit out of my depth in talking about this, but suffice to say the average person in China will strongly argue that Taiwan is part of China and any suggestion otherwise could be considered offensive.