Examples
Here are a few examples from projects that have adapted this work.
The BeeWare Project, a collection of projects used to develop, debug, and launch Python software, was the first to adopt the imposter syndrome disclaimer in their excellent documentation for first-time contributors. Their resource covers the disclaimer, how to help, using GitHub, setting up the environment, common questions, and other first-timer friendly resources. Definitely check it out if you're developing your own guide!
Netflix's Cloud Security team released Diffy: a differencing engine for digital forensics in the cloud, in July 2018. (Announcement on their tech blog) They're using this Imposter Syndrome Disclaimer in their Contributing Guide.
Cyrus IMAP, an email, contacts and calendar server, was another early adopter of this work. Take a look at their version. Their Contributor Guide is comprehensive.
MetPy, a Python Package for Meteorological Data, uses it here. Take a look at their contributing documentation.
The yt project, a toolkit for analysis and visualization of volumetric data, uses it here. Take a look at their terrific contributing documentation.