Cool - which country? Could you link to the Github repo?
Edit: the transition appears to be more like 2016, and one of the commits then refers to a file generated from COBOL: https://github.com/Skatteetaten/trekktabell/commit/5181d86c5...
> IDENTIFICATION DIVISION. PROGRAM-ID. FT7P200T. AUTHOR. PER J. RISTUN. DATE-WRITTEN. NOVEMBER 1993 ---------------------------------------------------------------- * BESKRIVELSE : PROGRAMMET BEREGNER FORSKUDDSTREKK FOR ÅR 2017 * (...)
If memory serves, they updated the program for each year (I guess only numerical constants and small rule changes), but they probably didn't update the source in the documentation.
(I just googled GP's email address (it's in their profile).)
Here our income tax declaration is very simple, the tax office already knows how much salary we have earned, and there aren't many deductions, so most people either don't have to file taxes at all, or just click next a few times and get their tax returns after a week or so. For 90% of people it takes less than 5 minutes.