Read A Better Way to Keep Track of Your Tasks? Check! by Dan (Simplenote)

Simplenote is great for keeping your life organized without getting in your way. Now, it gives you a whole new way to stay on top of your tasks: Checklists! Here’s one in action: Adding a checklist in the macOS app.
On our mobile apps, you’ll find a new button in the editor toolbar to add a chec...

Spurred on by a note from Joseph Dickson, I went over to Simplenote and discovered that indeed, they do support checklists! Sadly, there’s no source code for the web app in GitHub though they do have some code for the individual mobile apps.​