Neither [Scrum (as of the November 2020 Scrum Guide)][1] nor the [Manifesto for Agile Software Development][2] say explicitly that phases should be avoided. However, both have underlying principles and intentions that make phase-based approaches less attractive. The Manifesto calls for "early and continuous delivery of valuable software", where the software is delivered on a timescale "from a couple of weeks to a couple of months" (note that this timescale is from 2001 - some organizations are much faster today). Scrum calls for a Scrum Team that contains "no sub-teams or hierarchies" that are "cross-functional" and "self-managing".

The Agile Methods, including Scrum, call for rapid delivery of working software. Scrum, specifically, demands a working and potentially deliverable product Increment at least once per Sprint (and a maximum Sprint duration in Scrum is one month, so at least once per month). One way to ensure that you have a deliverable product at this frequency is by eliminating waste in the process, and the lean methodologies provide insight into how to eliminate waste.

It's well-documented that working as a team to identify and correct problems is more efficient. One of [W. Edwards Deming's 14 Points][3] is to "break down barriers between departments". Another one of the 14 points was that it's more expensive to depend on inspection to ensure quality and that building in quality from the beginning leads to a higher quality product at a lower cost. Taking a "finished" product and handing it off to a quality assurance team to test is a form of downstream inspection. These ideas can also be found in [The Toyota Way][4], which informs the Toyota Production System and the Toyota Product Development System, which paved the way for modern lean methodologies, including Lean Software Development.

Although it may be possible to meet the minimum expectations of Scrum and Agile with phases and handoffs, there's plenty of evidence from a number of domains that suggest that eliminating handoffs can improve quality and increase delivery speed.

Just because there are no handoffs doesn't mean that there isn't a need for specialists on a team. For example, the [Three Amigos][5] approach to refinement moves tester involvement early on in defining the work to be done by the team. There are also opportunities for developers and testers to pair during the development of the work, allowing for knowledge to be shared and quality built-in during development.


  [1]: https://scrumguides.org/
  [2]: https://agilemanifesto.org/
  [3]: https://en.wikipedia.org/wiki/W._Edwards_Deming#Key_principles
  [4]: https://en.wikipedia.org/wiki/The_Toyota_Way
  [5]: https://www.agilealliance.org/glossary/three-amigos/