XSLT is an integrated part of the web platform: browsers can load XML documents that use an XSLT stylesheet, and even inside HTML documents XSLTProcessor is available.
FTP was never integrated: it just so happened that some platforms shipped a protocol handler for it, and some browsers included an FTP protocol handler themselves. But I don’t believe you could ever, say, fetch("ftp://…").
Flash, like applets, was even more clearly not part of the web platform. It was a popular third-party extension that you had to go out of your way to install… or wait for it to be installed by some shady installer Adobe paid off. Though I have a vague feeling Chrome shipped with Flash at some point? I don’t remember all the history any more, this is a long time ago.
Older versions of TLS is definitely a more interesting case. It’s a different kind of feature, but… yeah, I might consider it.
<keygen> was an interesting concept that in practice went nowhere.
> FTP was never integrated: it just so happened that some platforms shipped a protocol handler for it, and some browsers included an FTP protocol handler themselves. But I don’t believe you could ever, say, fetch("ftp://…").
I never tried, but i believe the relavent spec said it should work, until it was deprecated and removed from the standard https://github.com/whatwg/fetch/pull/1166
With flash - that might all be true, but there was a time when many websites required it. It might not have been a de jure standard but it was a de facto standard. To the point where a browser not supporting it was considered broken. Apple refusing to support it was incredibly controversial at the time.
Fetch API is a pretty recent addition to the web platform. Back in the day, you could absolutely embed images of stylesheets from ftp: URLs. You could even use it with XMLHttpRequest (predecessor of Fetch). Even further back, gopher: was integrated with the web. URL schemes were invented for the web with the idea that http: is not the only one. These other protocols were really part of the web until they weren’t.