Also Simplenote from the Automattic folks: https://simplenote.com/developers/
Why would a notes app dictate what content can be stored in it?
It’s disclaiming. They are saying they do not believe their cloud-first tool is the appropriate place for that sort of content.
Simplenote is run responsibly by serious people. They are telling you what is absolutely common sense, in my mind: it’s not normal to store sensitive information on cloud notepad services you do not (and cannot!) pay for.
I suspect the reason is that their continuous-sync process (which is like SubEthaEdit etc.) does not work with notes encrypted at source.
Apple Notes makes more of this but it’s provided by a firm with several orders of magnitude more money and resources behind it.
They are trying to tell you not to do something risky (for you and for them I guess).
Complain about it on some abstract rights-based level all you like if you feel that it's a strange "concerning" issue. They are good people and they are doing this for the goodwill only (really: read the story about how Matt Mullenweg bought the simplenote.com domain for $29,000 and gifted it to them before he even discussed buying them out).
Me, I just won't use it for sensitive stuff.