First Wikidata doesn't work in practice according to the principle of not "restricting-the-world". It currently restricts users from adding new label in languages that they don't declare to know. It currently restricts users from creating sitelink to redirect pages.

It seems that the post you linked to explicitly says that it describes a personal opinion. To the extend that it's official policy, how does the process of appealing that design decision look like? Where could I go to start a discussion about a possible change?

In T132704#2270684, @ChristianKl wrote:

First Wikidata doesn't work in practice according to the principle of not "restricting-the-world". It currently restricts users from adding new label in languages that they don't declare to know. It currently restricts users from creating sitelink to redirect pages.

These are not parallel cases and implying they are is a bit offtopic.

It seems that the post you linked to explicitly says that it describes a personal opinion. To the extend that it's official policy, how does the process of appealing that design decision look like? Where could I go to start a discussion about a possible change?

Personal opinion from the ex-grand designer of Wikidata (prior to his election to the WMF board)... You're welcome to start a discussion at WD:PC, I would guess, but be prepared with some very good arguments--you are unlikely to be heard seriously otherwise. It would be better to accept that this is the case and wait for the Wikibase quality extensions to be enabled--which will be able to indicate to the user that there may be a problem with his selection, based on the current existing constraint system.