I know of two Intel <abbrtitle="Management Engine">ME</abbr> rootkits that didn't involve Intel <abbrtitle="Active Management Technology">AMT</abbr>; the latter can be enabled/disabled on "<spantranslate="no">vPro</span>" chips. One rootkit was from 2009 and seems less relevant now; the more recent of the two was by <spanclass="h-cite"itemprop="mentions"itemscope=""itemtype="https://schema.org/PresentationDigitalDocument">{{<indieweb-personitemprop="author"first-name="Mark"last-name="Ermolov"url="https://www.blackhat.com/eu-17/speakers/Mark-Ermolov.html">}} and {{<indieweb-personitemprop="author"first-name="Maxim"last-name="Goryachy"url="https://www.blackhat.com/eu-17/speakers/Maxim-Goryachy.html">}} at Black Hat Europe 2017: {{<cited-workurl="https://www.blackhat.com/docs/eu-17/materials/eu-17-Goryachy-How-To-Hack-A-Turned-Off-Computer-Or-Running-Unsigned-Code-In-Intel-Management-Engine-wp.pdf"name="How to Hack a Turned-Off Computer, or Running Unsigned Code in Intel Management Engine"extraName="headline">}} (<spanitemprop="encodingFormat">application/pdf</span>)</span>.
Without <abbr>AMT</abbr>, they required physical access. Most PCs are woefully unprepared against the sorts of attacks enabled by physical access, and <abbr>ME</abbr> is only one entry in a long list of issues.