tl;dr: You can confidentially include private repo activity on your GitHub timeline. This shows that you code more frequently, which tends to make a good impression. See screenshot here for details.
Even though first impressions can be deceiving, they still tend to have a significant impact, so it's important to put your best foot forward.
Github's timeline is a rough approximation of how much you've been coding in the past year. It's not an accurate measure in all cases because it doesn't include GitLab, BitBucket, and other VCS activity, but since GitHub has the largest market share, subconsciously people do correlate your GitHub timeline to general coding activity.
If you know nothing about this developer and this is the first (and only) data point about them, what would you think?
- How experienced to they seem?
- Do they seem to code often or just sporadically?
This screenshot was taken on the same day of the same developer's timeline with the Private contributions option enabled. Would this change your impression of them?
- How experienced to they seem?
- Do they seem to code often or just sporadically?
This simple change reveals 146x more coding activity in the past year. It makes a much better first impression than the first screenshot.
It seems GitHub might hide private contributions by default, so you may need to enable this manually. Don't worry, the details of the private contributions are always kept confidential. Nobody will see the repos, commits, PRs, or reviews you did if they related to private repos. GitHub found a nice balance of showing how often you use the site, but keeping sensitive details confidential.
To check, just open your GitHub profile in an incognito window and see how it looks to strangers.