RTF file support was removed in v6 (or rather reduced to read-only), with the developer telling one disappointed AppStore purchaser it is an "obsolete" format. I find this extraordinary.
I don't use markup. Nobody I know uses markup other than devs and coders. I don't care how "simple" it is to learn, who wants to learn all that syntax in the first place?!?
I'd buy FSnotes in a heartbeat if it still offered RTF format creation. The lack of formatting and images is the one thing that's starting to wear thin with the mighty SimpleNote. Even nvALT has been parked in favour of markdown-based nvUltra 😫.
Some people love markdown for sure. But they strike me as the kind of folk who eschew WYSIWYG apps in favour of doing everything in the command line, because they can. And I've been that guy too, but then I grew up!
The fashion for favouring markdown over RTF is the least Mac-like move I've seen in decades. It's the antithesis of "It just works". Yeah I get that platform cross-compatibility is a big deal, but I already got that with Simplenote. I just don't get images and formatting. Frankly I see no advantage of this app over Apple Notes. So I won't be spending money on FSnotes because if I have to learn new language and syntax, I'd rather put that energy into learning Welsh or Klingon than learning markup just so I can use a notes app.