As of now https://www.wikidata.org/wiki/Q18644427 (obsolete Wikidata property) has two subclasses, https://www.wikidata.org/wiki/Q60457486 (Wikidata property for a discontinued website; 59 properties use it as "instance of") and https://www.wikidata.org/wiki/Q108951239 (Wikidata property for a former identification scheme; 26 properties use it as "instance of"). Out of 9742 properties, 6920 have the datatype external-id; 85 properties, nearly in all cases with datatype external-id (about 1.2% of the external identifiers), are obsolete and their number will obviously grow.

I propose:

1. to reflect if, in order to mark these external IDs as obsolete, using "instance of" with the two aforementioned values is the best option or if we need a different way to mark obsolescence
2. to create a subsection Obsolete identifiers in the present section Identifiers for storing these obsolete external IDs (the external IDs going into this subsection could maybe be managed through https://www.wikidata.org/wiki/MediaWiki:Wikibase-SortedProperties)
3. to make the subsection Obsolete identifiers collapsible and to give registered users the possibility to autocollapse it through Preferences > Appearence (or > Gadgets)