The GitLens FAQ mentions:
Yes, all features are free to use on all repos, except for features:
- marked with a ✨ require a trial or paid plan for use on privately hosted repos
- marked with a ☁️ require a GitKraken Account, with access level based on your plan, e.g. Free, Pro, etc
The Commit Graph feature is marked ✨.
So this is:
- either a bug (a fork of a public repository is public)
- or a fork is considered by GitLens as "private use" somehow.
As a test, try and create your own public empty repository, and import through a remote named "upstream" the repository you intended to fork: check if the Commit Graph feature is available then.
As the OP comments:
- firstly you clone via
git clone https://github.com/me/fork.git
, the extension sees that the repo is public,
- then you type
git remote set-url origin git@github.com:me/fork.git
and the extension continues to work.