The launch of the 2020 version of the Scrum Guide reimagined the role of the Scrum Leader. Servant-leader a term that had become synonymous with the role of a Scrum Master was coined by Robert Greenleaf in his essay published in 1970.

A servant-leader focuses primarily on the growth and well-being of people and the communities to which they belong. While traditional leadership generally involves the accumulation and exercise of power by one at the “top of the pyramid,” servant leadership is different. The servant-leader shares power, puts the needs of others first and helps people develop and perform as highly as possible.

This style of leadership worked well for Scrum, where the importance of a Scrum Master is not in lieu of any designation and influence enjoyed by him/ her is drawn from the team itself.  

Jeff Sutherland: The 2020 Scrum Guide is shorter, more focused, and has One Team. In addition to tying the three artifacts to goals with commitments, we addressed two of the biggest challenges in the industry: servant leaders who don’t lead, and self-organizing developers doing whatever they want and not meeting the commitments.
After wrestling for a long time with servant leadership, we finally just changed the word order. A Scrum master is a leader who serves.
Ref: Changes in the 2020 Scrum Guide: Q&A with Ken Schwaber and Jeff Sutherland

One of the major catalyst for certain changes made to the Scrum Guide was the Comprehensive Human Appraisal for Originating Software Report published in 2019. It stated that 58% Agile projects failed or were challenged. And one of the major reasons for this was attributed to Scrum Masters not leading the teams.

Today, the Scrum Master needs to be a leader first. Anyone in the organization, who delivers value, enables the team and embodies the principles of Agile is a Scrum Master.

Scrum Master - A Leader who Serves | From Servant Leader to True Leader - Scrum Guide 2020: