The form to archive an extension or skin contains the following task:
- Archive Wikidata item associated with documentation page (provide URL) (remove nearly all statements, except instance of (P31) = MediaWiki extension (Q6805426)/MediaWiki skin (Q21996535), which should add a qualifier end time (P582) = the YYYY-MM-DD date that you decided to archive extension/skin, generally per edit history, change English (and other languages if possible) description to "archived MediaWiki extension"/"archived MediaWiki skin" (and its translation)).
I find it irritating that all the data of archived extensions or skins are cleared from Wikidata. All the data is still valid or were valid at some point in time in the past. Instead of clearing the Wikidata item of the respective extension or skin it should be marked as abandoned instead like it was done e.g. here.
The situation is comparable to updating information of a person that just died. Nobody would come up with the idea to clear all Wikidata facts. Instead they are marked as deceased and a date of death is added. There is no difference when it comes to a software I believe.