There are downsides with downloading their app just to input bad data, but it's a fun thought.

I posted in !opensource@programming.dev to collect recommendations for better apps

The post: https://lemmy.ca/post/32877620

edit, while we're at it we might as well offer an alternative app to people

Leading Recommendation from the comments

The leading recommendation seems to be Drip (bloodyhealth.gitlab.io)

Summarizing what people shared:

• accessible: it is on F-droid, Google Play, & iOS App Store
• does not allow any third-party tracking
• the project got support from "PrototypeFund & Germany's Federal Ministry of Education and Research, the Superrr Lab and Mozilla"
• Listed features:
  • "Your data, your choice: Everything you enter stays on your device"
  • "Not another cute, pink app: drip is designed with gender inclusivity in mind."
  • "Your body is not a black box: drip is transparent in its calculations and encourages you to think for yourself."
  • "Track what you like: Just your period, or detect your fertility using the symptothermal method."

Their Mastodon: https://mastodon.social/@dripapp