It's also in CLDR supplemental data: https://unicode.org/reports/tr35/tr35-dates.html#Calendar_Data

On Fri, Mar 16, 2018 at 9:43 AM, Florian Weimer <fweimer@redhat.com> wrote:
Would the tz project be interested in keeping track of the Japanese Era information?

glibc currently stores it as part of its locale data, but I think this causes some issues (particularly with downstream builds).

Thanks,
Florian