[drip.](https://bloodyhealth.gitlab.io/) [source](https://gitlab.com/bloodyhealth/drip)

  - around since 2019. Last update 2 months ago
  - iOS, Android
  - React Native

Mensinator [source](https://github.com/EmmaTellblom/Mensinator)

  - around since 2024. Last update 2 weeks ago
  - Android
  - Kotlin

[Menstrudel](https://menstrudel.app/) [source](https://github.com/J-shw/Menstrudel)

  - around since 2015. Last updated 3 weeks ago.
  - iOS and Android
  - Dart

[Tyd](https://unobserved.io/tyd/) [source](https://github.com/unobserved-io/tyd)

  - around since 2023. Last updated 2 years ago.
  - iOS
  - Swift

EDIT: Someone else pointed out this closed-source alternative that got a 92% by ORCHA: https://www.my28x.com/

I think the biggest thing I'd like to see is a data format standard defined. You should be able to "take your data with you" and go anywhere you like. If you decide an app is unethical or if your favorite OSS app stops being updated, it should be simple to switch. Many apps let you export your data. Maybe someone can make a converter between popular proprietary apps and a common data structure spec