Thank you for saying outright what you're looking in the tagline.
Let me in response outline what my thought process was in the moment writing it. Please take it as a humble personal opinion, nothing more.
Your notes: on every device, private, secure, forever
I wrote this generic tagline because it kinda suited your stated needs yet there are few things I would like to add to the discussion.
Before submitting this tagline I asked a few non tech friends what they think. The response I get was that they need some kind of attention grab and descriptive "value" to hook them in.
So here's the thought process I went through:
I believe technical users will find this app anyway. They have the tools, patience to study the program because it is essential thing they going to invest a lot of their time.
1.1 I believe there's no way, we could convince technical users with descriptions of the product in every paragraph. They need to study it anyway and they can use all the docs for that. So to cater to them it is great idea to improve docs (for example by clipping good responses on this forum into bite sized pages on github)
I indeed didn't want word "free" in the "description" because if you ever intent to provide a paying service (like Joplin Cloud with note collab feature), you'd be accused of lying by mostly non-technical customers. It's hard to explain paying features to them if you're sold them on the premise of being free.
2.1 Non tech people don't understand and don't care about technical jargon.
For example: what FOSS means, they don't get what is source code, they never seen it anywhere.
They don't know what markdown is, they just want to make font bigger or smaller. How we do it — is of little concern to them.
I believe this app need more non tech users. On this forum we've got already biased look of the population — people who had the patience to keep up with the app's forum. Tech users are already sold on the program without much marketing. I believe tagline, photos, descriptions are needed for those who cannot be expected to study the docs and forums. Although I wouldn't want to left out the tech users but I just wanted to point out that there are channels of communication and they are a lil different for different audiences.
I actually chose not my own tagline in the poll but this one because it has playful, easy-going attitude, like brain candy for a user idly browsing an app store.
Secure, free, and anywhere you want it to be
In summary, if it is required to show in tagline at least some functionality, I'd choose the ones non tech people actually care about. And in process I'd try not to shoot myself in the foot.
By the way, @laurent I think you're doing a great job in uniting these two audiences, yet I thought I could shift your attention from features of the app and cater a lil more to non tech folk. I think you're on the great track with this "Do X, get Y" guidance.
I don't know personally what action we could reliably sell to regular folk who makes groceries lists and writing poem drafts in the app — we don't send anyone your data (privacy/loyalty)? you could pass on your data to future generations (future proof)?
if I get the time, I may attempt to write some more taglines, yet I imagine it is already quite exhausting to constantly be choosing the best ones among the waterfall of new ideas.