The Deadpendency report is a GitHub check run. The bulk of the report lives in the check run body, which has a (currently undocumented) limit of 65335 characters.
When a repository has enough dependencies in some cases it will exceed this limit. In this scenario Deadpendency will remove non-key information from the report. Currently, this means that passing dependencies will no longer include links to the package registry or source repository.
In some cases, this limited report will still exceed the maximum report size and will appear truncated.
Longer term, the Deadpendency report will be available external to GitHub to avoid the problem entirely.
Deadpendency makes a number of requests to external services, most notably GitHub. When a single repository has too many dependencies it is not possibly to reliably retrieve the dependency information without hitting rate limiting issues.
As such Deadpendency will reject the request when a repository has over 500 unique dependencies.