You're right; I just tested it in Discord, which does have a spell checker, and it seems to use the system dictionary (but it seems one way when teaching it new words), but it's still missing other global text field functionality: https://i.imgur.com/qpOU9dz.png
The point remains: Electron apps generally lose many of the features that the host OS gives native apps for free.
But the native web, used properly, includes many of them itself. Things like accessibility, metadata, etc. You only really lose features if you're circumventing native web functionality.
Sometimes web developers will circumvent core HTML features because they don't do everything they want and aren't extensible enough, although this is generally considered bad practice if it's at all avoidable. Sometimes they just don't bother to write semantic HTML even when they could. But the platform itself has the infrastructure; it's possible to write bad software on any platform.
Kind of odd, is doesn't look like there's a built-in spell checker available in Chromium since you need a spell check provider[0]. I imagine Google injects it into their Chrome when deploying.
