Some examples I found:
- Green Software Foundations' various project github repo discussion groups - https://wiki.greensoftware.foundation/governance/projects
- Cloud Native Computing Foundation Environmental Sustainability Workgroup's github discussion group - https://github.com/cncf/tag-env-sustainability
- Sustainable Living at Stackexchange - https://sustainability.stackexchange.com (the green computing related posts are more limited in numbers it seems)
More suggestions and opinions is most appreciated!