The 2.1.0-3 system version released on June 15, 2011 for the original Nintendo 3DS. The official patch notes for this firmware notes "Improvement to system stability and convenience."
Beginning in early 2016, 2.1.0-3 was installed onto the system's NAND as a necessary installation step for arm9loaderhax[1], to retrieve a console-specific OTP dump. As this firmware naturally didn't release for any console aside from the CTR-001 Nintendo 3DS, installation on newer devices nets known issues such as corrupted screen data, softlocks, and potential to software brick on FTR-001/Nintendo 2DS units upon formatting the console.