In this particular case, the character string involves the Italian flag emoji along with characters in the Sindhi language, and it appears the system crash happens when an incoming notification is received with the problem-causing characters.
Based on information shared on Reddit, the character string began circulating on Telegram, but has also been found on Twitter.
These kind of device-crashing character bugs surface every so often and sometimes become widespread, leading to a significant number of people ending up with a bugged iPhone, iPad, or Mac. In 2018, for example, a character string in the Telugu language circulated around the internet, crashing thousands of devices before Apple addressed the problem in an iOS update.
There is often no way to prevent these characters from causing crashes and freezes when received by a malicious person, and crashes caused through notifications often cause operating system re-springs and in some cases, a need to restore a device in DFU mode.
MacRumors readers should be aware that such a bug is circulating, and for those who are particularly concerned, as this bug appears to impact notifications, turning off notifications may mitigate the effects. Apple typically fixes these character bugs within a few days to a week.