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IMO it's easier to have it as a single year, the most recent year, rather than a year range as Yomichan had it; more consistent across all files.

I believe the FSF recommends include each year in the copyright notice: https://www.gnu.org/licenses/gpl-howto.html And in general, copyright notices are supposed to at minimum list the year of creation. I.e., you are not really supposed to modify them, though you can add to them. I think it's fine to stick with it as it doesn't really add much burden.

I've also wondered a few times if the combined copyright of yomitan+chan should just be grouped into something like

Maybe... but I found it a little weird to do this when the project is a distinct fork. My understanding is that you must retain original copyright notices when you fork, and add your own.

But dunno, maybe I'm being too anal. 😄

I'll update the dates to a year range then; the only burden is when someone has to copypaste it to a new file I suppose. Tbh I don't think I've ever read that link you shared, hence my suggestions not really being in line wtih it.