It seems this is still an active bug (or possibly misfeature depending on your perspective). Support for pasting (WebKit bug #49141) is apparently implemented in both WebKit upstream and Safari, but it seems to have been done so in a way that is not compatible with anything (WebKit bug #170449).
The BugZilla for the latter bug suggests that there is an associated radar within Apple about this.
In other words, I guess the straight answer to your question is "wait for either Apple to conform to what everyone else is doing, or for support for Safari's way of doing things to percolate everywhere", whichever happens first.
Edit: It seems to consistently work for me in Safari Technology Preview (Release 52 (Safari 11.2, WebKit 13606.1.9.3.1)) under High Sierra, including on both Trello and Facebook.