I used to use MAFF (Mozilla Archive Format)[1] a lot back in the day. I was very upset when they ended the support[2].

I never dug deeper whether I can unzip and decode the packing, but saving as simple ZIP does somewhat guarantee future-proofing.

[1] https://en.wikipedia.org/wiki/Mozilla_Archive_Format

[2] https://support.mozilla.org/en-US/questions/1180271