Absurdly, comically simple is the way to go. In these situations I usually just send myself an email, or write something down in the current physical notebook.

About 20 years ago I spent a while writing a server to do a bunch of automation and reminding and so forth, and found that I was spending more time on that thing than I was doing real work, so I ditched it, just like I did OneNote and all the other apps. There's no need to fix bugs in a text file, nor do you need to upgrade your version of Python or whatever.

Simple will survive decades with zero effort. Complicated pieces of code (especially ones that use networking or browser tech) will need to be maintained, patched, debugged and ported. Life is short. I'd rather actually be productive than have to spend time keeping some productivity app running, or worrying if the vendor of the online notes app I chose to use is going to be bought by Oracle or something.

You're making me think that the even absurder method is to use mail drafts as your files. Everything is synced between all your devices, you have built-in modification date, a full editor in the web version or applications for a more integrated experience, high portability and backup (transferring from IMAP account to IMAP account is easy), labels and/or folders depending on your provider, efficient search, attachments...

Actually in the past I used my email account as my RSS reader. It had integrated read/non read status, mass "mark as read" as needed, was synced on all platforms I cared about without anything to install. I didn't really changed my subscriptions a lot so configuration was not complicated.

Sometimes a webmail is just a good enough UI for text-related things.

Set up an email alias that cats (appends) your note to your org file.

Then, just send your notes (via sms) to the email.

If you version control it using Fossil, and it's in markdown, you can access it directly from the repo website.

You can with Github as well. And make changes to it via the website. Not on mobile though, unless you switch to the desktop site.

But you can't make changes to it online.

Depends. You're right if you use a conventional version control approach where you write on your local machine and then push to the server. But that's not the only way to use Fossil to store your notes.

If you're using the wiki (using one or more wiki pages) everything is entered into the browser whether online or using the built-in server running locally. Other options are to create an issue for each note or create a new forum thread for each note. The nice thing about these options is that you get the search and querying power of an sqlite database applied to your notes for free.

Disclaimer: I haven't used this approach because I don't use mobile devices for note taking. Just pointing out that it is possible.

On Android, I've been using Markor with Syncthing for years now and never had any issues.