I recently stumbled on Calendar.txt, by author and teacher Tero Karvinen. It's a text document based on a similar ...

The new calendar button will appear at the top of emails, right next to the summarize button that arrived last year. The calendar option will show up in Gmail threads with actionable meeting chit-chat ...

Gina Trapani, founder of the website you're reading right now, popularized a plaintext format for to-do lists way back in 2006 (please excuse any weird formatting on that nearly 20-year-old article).