Among other findings, an EY Center for Board Matters Report, "Audit Committee Report to Shareholders in 2017," showed a sharp rise (from 15% in 2012 to 84% in 2017) in the percentage of audit committees that explicitly state that the committee considers non-audit fees and services when assessing auditor independence.
As we know SOX is 15 years old, and the requirement for audit committees to assess the auditor's independence, including nonaudit services, is not new. So, what's driving the dramatic increase in disclosure? Are audit committees merely stating something they've been doing all along? Is it the constant drumbeat from SEC and PCAOB leadership about nonaudit services' impact on independence and that audit committees should step up, or something else?
And the inclusion of nonaudit fees as an independence consideration... there is no explicit SEC independence rule under SOX outside of proxy disclosures. The closest to an SEC independence "rule" is a position the staff took in the 1994 "Staff Report on Auditor Independence" (not very widely known, I suspect), which posited that deriving more than 15% of firm revenue from a single client (or group of related clients) raised questions about independence. That said, certainly fees would also need to be considered under the Rule 201-b's "general standard" (under Reg S-X).