This is about creating the necessary infrastructure required for a new parser function that outputs "rich wikitext" with links, in contrast to the "escaped wikitext" the existing property parser function outputs.
|mediawiki/extensions/Wikibase : master||Link external ids when used in Wikitext|
|Resolved||thiemowmde||T142940 Create client functionality for getting human readable wikitext from Wikibase statements|
|Resolved||hoo||T148025 [Task] Allow formatting of external identifiers as wiki links when using rich wikitext output|
It is a breaking change to the FORMAT_WIKI output indeed. But given this is not used by us anywhere currently, the impact is null.
The only way to get FORMAT_WIKI output currently is to use the wbformatvalue API. This is not being used as far as we know (see T147591: Look at FORMAT_WIKI usages in the wbformatvalue API for that), but nevertheless we still announced this change (https://lists.wikimedia.org/pipermail/wikidata/2016-October/009802.html etc.).