We have G178 which indicates adding a control that provides incremental levels is sufficient - but the test procedure doesn't seem to require incremental support.
https://www.w3.org/TR/2016/NOTE-WCAG20-TECHS-20161007/G178

Is it acceptable for a page to add a control for resize of text but only support 100% and 200% without incremental levels? If incremental levels are required - how many?