From quick testing, WV, WN, WT, WB don't work, but Meta and MW do. Not a very high priority though.

I don't follow this entirely. If I try parsing the following wikitext in the enwiki sandbox, they link to "en:Foo", "fr:boo" on enwikisource and "de:Berlin" on wikivoyage ... so the syntax proposed above is definitely not right.

[[wikisource:en:Foo|foo]]
[[wikisource:fr:boo|boo]]
[[wikivoyage:de:Berlin|Berlin]]

Similarly, the following two link to "frwikisource:boo" and "dewikivoyage:Berlin" on enwiki and not to frwikisource or dewikivoyage pages.

[[frwikisource:boo|boo]]
[[dewikivoyage:Berlin|Berlin]]

So, I don't follow the request yet. Can you clarify the right syntax?

First off, I thought the external link -> interwiki link code was in VisualEditor? Did it get moved to Parsoid?

See https://en.wikipedia.org/w/index.php?diff=726361747&oldid=724348641

In that example, both ru.voy and de.voy are not on en.wp's interwiki map - they are on en.voy's map and the chaining of interwiki links is what make those work. Your en.voy example is flawed, because it was a link to just https://en.wikivoyage.org, which is not actually a wiki page, and cannot be represented as an interwiki link. ("[[voy:]] would link to https://en.wikivoyage.org/wiki/). Using an actual page title appears to work fine: https://en.wikipedia.org/w/index.php?title=User%3AWhatamidoing_%28WMF%29%2Fsandbox&type=revision&diff=726721222&oldid=726679055

There seem to be multiple questions, so let me know if I missed anything:

1. I actually want [[voy:|Label]] instead of [https://en.wikivoyage.org Label].
2. I prefer [[s:fr:Foo|Label]], but I don't know whether that one is theoretically more correct than [[frwikisource:boo|Label]].
3. I have no idea whether this is Parsoid's or VisualEditor's problem to solve.
4. But I do know that it's broken on office.wiki, and it sounds like it's mostly broken on other sites, too. Perhaps we need a separate task for "update the interwiki maps at hundreds of WMF wikis"?

Some thoughts:

• I don't think we need to update the interwiki maps to get this functionality
• Parsoid can add additional logic to chain prefixes of the form <project>:<wikiprefix>:<title> like voy:it:Paris or s:fr:Foo ... this will be a bit involved but can be done. However, this will break HTML -> HTML roundtripping. So, (assuming we are on enwiki), http://fr.wikivoyage.org/wiki/Paris will become [[voy:fr:Paris|Paris]] and on parse, this will generate http://en.wikivoyage.org/wiki/fr:Paris which is a different url compared to what we started with. This is a subtle change, but we could update our testing to normalize for this perhaps.

How important is this functionality?