Acabei de receber uma resposta do suporte do Slack sobre o mesmo problema. Ela disse:
Thanks for getting in touch, and I'm sorry for the concern generated with this logs issue — this is a known bug. It was caused by a Slack outage that we experienced a couple of weeks ago.
We'd never seen anything like this before and so I can only apologise that they suddenly started taking up so much space. We've got a bug report filed to improve on this in the future. I can let you know once we've implemented a fix if you like?
In the meantime, if you go to the menu and select Help > Reset App Data, you should see those files disappear. Please note, that this will sign you out of your workspace, so you'll need to log back in.
Acabei de optar por excluir manualmente os arquivos de log por enquanto e aguardarei a correção.