In these sort of situations, I always find it very helpful to check MDN. In this case, you can check the articles for linear-gradient, background-clip and -webkit-text-fill-color. Near the bottom, there is always a section titled Browser compatibility. It lists, in a table, browsers that support the CSS rules and in footnotes goes into specifics. For example, about the -webkit-text-fill-color, it says that
 This feature is implemented behind the preference layout.css.prefixes.webkit, defaulting to false. Since Gecko 49 (Firefox 49.0 / Thunderbird 49.0 / SeaMonkey 2.46) the preference defaults to true.
This should answer your question on why the -webkit- prefixed version is supported, and since when. Also note that at the top, the article mentions
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
In this particular case, you might be able to get away with what you want to do, at least in Firefox and Chrome. Other browsers... that's trickier.
Finally, "is there any list of foreign vendor prefixes support (like -webkit- prefix support in FF)?" Sort of. Again, MDN is usually very complete and up-to-date. Hope that helps.
The same is true of IE11 and Microsoft Edge. I'm still bitter that things had to come to this, but it is what it is.