Should there be Slack in Scrum?
The scrum guide does not mention anything about slack. Does this mean that the teams are being given the freedom to allocate a certain percentage of their capacity as slack?
In my opinion, some amount of slack is important to ensure the team is moving at a sustainable pace.
What do others think about this?
Teams can implement whatever strategy they think best in order to sustain a product.
Suppose a team had a pull-based workflow strategy. They might then plan their capacity so the rate of work entering and leaving are observed to match and WIP does not build up. To manage this successfully, slack in the workflow may be implied. The team’s plan, however, would be based squarely on evidence.
Scrum guide doesn't mention/encourage slack in scrum team but somehow it happens for unexpected reasons.
Team's velocity seems include slack. As a scrum team, they should improve it sprint by sprint to be a self-organized team and productive team.
One way or another, teams are extensions of individuals.
Do we have slack in our day-to-day? I'd argue most of us do, so why can't teams have slack, too? Nobody's a robot (at least not now :) )
Agile processes promote sustainable development. The sponsors, developers, and users should be able to maintain a constant pace indefinitely.
That is one of the principles behind the Manifest for Agile Software Development.
Although slightly different (and rooted in a design and manufacturing environment), The Toyota Way promotes heijunka, or workload leveling.
Both of these concepts are about long-term sustainability and wellness of the people who are working.
Consider this: if you have no slack, you are at your maximum possible effort. How long can you sustain that pace before you grow tired? When you get fatigued, you begin to make mistakes. In terms of the Manifesto for Agile Software Development, you aren't able to give continuous attention to technical excellence if you are under extreme pressure. Going back to Toyota Way principles, by working at maximum possible effort, you are focusing on short-term gains instead of a long-term philosophy.
So - yes. You are absolutely correct. You need to ensure that your plans include accounting for things that come up. Perhaps high priority work that needs to be expedited. Perhaps it's unplanned absences from team members. Maybe it's just one or more individuals who need to slow down and keep focus (focus is also one of the Scrum values). All of these things need to be considered when planning your Sprints and all of them would be considered slack.