Alternatively, ensuring you have enough slack in the schedule is, at least for some tech leads and project managers, an essential tool to enable meeting deadlines.

(So, I suppose using "slack" in a positive sense by project management, while probably still being considered a pejorative thing by non technical management or beancounters...)

yep, having some slack is the only way for someone / something to able to respond to uncertainty. technically having firefighter on standby and policemen on patrol are a form of slacking, and we (should) have no problem with that.