Thanks for the review! I contacted the upstream author of hufilter and it's now distributed under CC-BY-SA-4.0 license. I renamed the other "pulic-domain" short description to "unlicense".
I'm not sure how to fix the other issues. According to https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/#copyright-field, the copyright field is for relevant copyright notices but a lot of the filters don't have any copyright notices. I would like to make the extension available to Chromium but I don't know how to get Chromium extensions installed system-wide. I can't find any other Chromium extensions packaged for Debian either.