This is one of those things that seem to be highly variable. Some seem to to get failure in months, while others get many years - 10 years being reported.
What does seem to help is getting USB2 drives (I suspect because they typically run cooler than USB3 ones) if you can find them nowadays. Also ones with metal casings can help with heat dissipation. Having said that I have some nano form factor ones that I have been using for years and they are too small to have much cooling.
Sounds as if you definitely want to get an automated backup of the flash drive in place going forward. The two standard ways of achieving this are:
via the appdata backup plugin which includes an options for the flash drive as part of scheduled backups.
Via Unraid connect which supports automated backups to the cloud (on Limetech servers).
There is still the manual backup method of clicking on the boot drive on the Main tab and selecting the backup option from there.