This might work OK for simple period tracking, but for more advanced fertility tracking, it’s not really possible to do pen-and-paper without being a huge pain.

For example, the open-source, local-only app Drip optionally uses a “sympto-thermal method” to estimate when a user is fertile; the app takes not just the days they are on their period into account, but also their body temperature, cervical mucus values or cervix values, and throws those inputs into an open-source algorithm for much more accurate results.

Doing all that on pen and paper might be possible, but it would also be a bit of a nightmare.

PrivacyGuides has a section about this on their website: https://www.privacyguides.org/en/health-and-wellness/#menstrual-cycle-tracking

At the time of writing, it seems Drip is the best option on both iOS and Android. The app is free, open-source, and all data is completely local. Honorable mention to Euki as well

Other than Fedora Atomic derivatives (like Bazzite) and of course SteamOS, the only big one I can think of is NixOS? It’s certainly not tailored to gaming, but it does look like 2.4% of ProtonDB users run it