"waterfall" is just a pejorative term for the kind of planning represented by a Gantt chart, which kinda looks like a waterfall.
This kind of planning is _necessary_ for traditional engineering projects where certain steps of the process can take months or years and requires scheduling of many resources.
Purely software projects usually don't need that because if your tools are good you can ship a completely new version in minutes to hours instead of months to years. You also get very easy do-overs if there are mistakes which you don't get if you're building a bridge or a factory.
This kind of planning is _necessary_ for traditional engineering projects where certain steps of the process can take months or years and requires scheduling of many resources.
Purely software projects usually don't need that because if your tools are good you can ship a completely new version in minutes to hours instead of months to years. You also get very easy do-overs if there are mistakes which you don't get if you're building a bridge or a factory.