Input modules and acquisition devices can group channels. The GUI visualizes the groups, the CLI application allows option specs that refer to groups. The srzip file format does not store the group information, so that this information is lost across save and load operations.
To be honest, I think we should rather focus on the new file format instead of keeping srzip around for even longer.
Although I reported against srzip v2 (that's what mainline has), I can agree to not change v2 assuming that v3 becomes available and supports the feature. Did a search for "group" in https://sigrok.org/wiki/File_format:Sigrok/v3 and there seems to be no support for channel groups either in the current outline. Unless I missed it since another phrase is used.
Had another look. Missed it the first time. :) There are "CG" related packet types, though their description is empty at the moment.