Are AM5 boards still frying 9800X3D CPUs?

There were reports involving ASRock boards in particular, with some reports involving ASUS too. That does not mean every board from either manufacturer is affected, or that other brands are guaranteed safe. The discussion does not establish how common failures are or whether every cause has been fixed.

For a new build, check the support page for the exact motherboard and CPU combination, install a current stable BIOS, and look into any recurring reports for that specific model. BIOS updates have been discussed as a fix, but there were also claims of failures after updates.

I would not count on undervolting as protection. There were reports of undervolted CPUs failing too, and the cause of every reported failure is not clear. If reliability is the goal, start with the board's supported default settings rather than assuming a lower voltage eliminates the risk.

If you want to minimize uncertainty, avoiding ASRock is one cautious choice given how often it came up in the reports. Some people also prefer to avoid certain ASUS models. That's a risk preference, though, not proof that a particular alternative cannot have problems.

CPU damage isn't the only compatibility issue worth checking. Memory stability came up separately in the discussion, so it's worth checking the board's memory compatibility information as well. A board having no reported CPU failures doesn't guarantee every RAM kit will work without trouble.