Probably OK, but access it from Electron (let alone fully-native apps) and now you may not technically not be OK anymore—is that functionally equivalent to a web browser? Hard to say. And much of the benefit of web APIs, versus just serving pages and HTML fragments, is being able to serve those kinds of heterogenous clients, or to allow access to 3rd parties, and who knows what they might use to access it, so... yeah, you can push low-usefulness (browser-only, first-party-use-only) web APIs through Cloudflare and you're likely in the clear, but go beyond that and it gets murky fast.
And even then, the web API thing is subject to the rest of the restrictions in that same section ("serving web APIs subject to the restrictions set forth in this Section 2.8") so "serving video or a disproportionate percentage of pictures, audio files, or other non-HTML content is prohibited" (emphasis mine) meaning that if too much of your traffic is JSON or protobufs or what have you, they could send you a nastygram or simply cut you off, though they might choose not to.
Personally, I'd not rely on Cloudflare's free or $20 plans past MVP/experimentation or hobbyist use, precisely because the terms are restrictive and vague. Too risky. Then again, what can you expect for nothing-to-peanuts prices?
And even then, the web API thing is subject to the rest of the restrictions in that same section ("serving web APIs subject to the restrictions set forth in this Section 2.8") so "serving video or a disproportionate percentage of pictures, audio files, or other non-HTML content is prohibited" (emphasis mine) meaning that if too much of your traffic is JSON or protobufs or what have you, they could send you a nastygram or simply cut you off, though they might choose not to.
Personally, I'd not rely on Cloudflare's free or $20 plans past MVP/experimentation or hobbyist use, precisely because the terms are restrictive and vague. Too risky. Then again, what can you expect for nothing-to-peanuts prices?