[kjagiello]

For sequence diagrams in text format you can take diagwiz for a spin. There is also an online version of the tool. Disclaimer: It is working, but it's still just an experiment and might change a lot.

https://github.com/kjagiello/diagwiz

https://diagwiz.io/playground/ (not really mobile friendly)

[smusamashah]

These do the same i.e. ASCII sequence diagrams

https://textart.io/sequence

http://weidagang.github.io/text-diagram/