Keep in mind that a lot of Slack’s employees —- especially early ones —- are Canadian, and Canadians don’t get unlimited PTO (by law, I think). So they are being told when they should be using some of their PTO days.
Canadian here. There is no law against unlimited PTO. There are laws about statutory minimums however.
Personally, I think most of the "western world" has a more healthy idea of PTO than the US. Some companies here are better than the norm, but a lot are not really great.