There is a new pull request by atk against master on the void-packages repository

https://github.com/atk/void-packages patch-2
https://github.com/void-linux/void-packages/pull/18427

slack-desktop: add missing dependencies
tar and xz are no longer in masterdir, so the conversion of this .deb package failed.

A patch file from https://github.com/void-linux/void-packages/pull/18427.patch is attached