There are at least two Famicom games that require decimal mode: the original Bit Corp. releases of Duck Maze and Othello. These games must have been developed on a Famiclone with a decimal-mode-bearing CPU. When run on an original Nintendo console, or an accurate emulator, these games glitch whenever a digit crosses the 10 mark. Here is an example: https://youtu.be/uCOt5GVpBc4
The Australian HES releases of these two games were updated to no longer require decimal mode, as HES must have insisted that these games run on original Nintendo consoles.
As for emulator support, one could just add an "Allow Decimal Mode" option in the emulator's settings, and insist that users enable it by themselves, just as they have to choose input devices by themselves. Theoretically, one could also somehow put this into the NES header, possibly as a Mapper or Submapper to Mapper 0, since both games use simple NROM boards. Using any other bits of the header just for two games would be rather wasteful.
The Australian HES releases of these two games were updated to no longer require decimal mode, as HES must have insisted that these games run on original Nintendo consoles.
As for emulator support, one could just add an "Allow Decimal Mode" option in the emulator's settings, and insist that users enable it by themselves, just as they have to choose input devices by themselves. Theoretically, one could also somehow put this into the NES header, possibly as a Mapper or Submapper to Mapper 0, since both games use simple NROM boards. Using any other bits of the header just for two games would be rather wasteful.