The 2.1.0 system version is a Old3ds version only. However, this version was used a lot because in this version is still possible to dump the otp in arm9 kernel mode due to a bug. So, this version was used to dump the otp.bin necessary to install (the now outdated) Arm9LoaderHax. This version has a lot of bugs, for example spider (a browser exploit in arm9 kernel mode), the colors has lots of weird glitches, and can brick Old 2ds and New3ds/2ds if you do the following:

If you format an old2ds you aren't able to complete the Setup because the system tells you to turn on the 3D, but since the 2ds doesn't have this feature you are in a softbrick that is recoverable with a copy of Cubic Ninja or with NtrBootHax. If you format a New3ds you will brick because the system will act as an old3ds. If you go in Sleep Mode on a New3ds, if you re-open the console the screens are still black and you have bricked