Tried it \o/
Two remaining issues I found:

• https://wikidata.beta.wmflabs.org/wiki/Special:ListDatatypes is missing messages for this
• and there is a missing message in the error message here:

Change 526770 had a related patch set uploaded (by Ladsgroup; owner: Ladsgroup):
[mediawiki/extensions/Wikibase@master] Add remaining i18n messages for EntitySchema data type

https://gerrit.wikimedia.org/r/526770

Change 526770 merged by jenkins-bot:
[mediawiki/extensions/Wikibase@master] Add remaining i18n messages for EntitySchema data type

https://gerrit.wikimedia.org/r/526770

A couple of issues:

On the beta system, Pages that link to "EntitySchema:E1" isn't showing anything, despite Entity schema (P253078) having it as a value of property P253078

Secondly, can we confirm that, on WDQS, a property Pxxxxx with data type 'EntitySchema' (eg this proposal Wikidata:Property proposal/Shape Expression for class will lead to statements of the form

wd:Q5  wdt:Pxxxxx  wd:E10

with the wd:E10 being appropriately linked by the UI and turned into an appropriate URL in the download ?

If this is not the case, then the datatype is not ready to go. (Also, are these a reflection of a fundamental issue that the internal type ought to be "Wikibase entity id", not "string" ?)

Ok. Based on the comments here and in project chat let's flip the config switch for this to off and see what we can do about the raised points.

In T214884#5394245, @Jheald wrote:

wd:Q5  wdt:Pxxxxx  wd:E10

I don't support reusing wd: prefix (T225778#5391086). We should define a new prefix like wdes:E123

(striken 2023-08-29; see T344609#9124706)
(reinstated 2023-10-18; see T344609#9260423)

@Bugreporter : T225778 "Define canonical URI for EntitySchemas" was opened a few weeks ago as a specific ticket for what the canonical URI should be, so I've added that in as a subtask for this ticket.

But, why is it that you object to wd:E10 (ie http://www.wikidata.org/entity/E10), similar to eg wd:L20 (ie http://www.wikidata.org/entity/L20) for a Lexeme ?

It refers to https://www.wikidata.org/wiki/Special:EntityData/E10, and Special:EntityData is served by Wikibase, but EntitySchema is not based on Wikibase.

(striken 2023-08-29; see T344609#9124706)
(reinstated 2023-10-18; see T344609#9260423)

Waiting on discussion regarding which direction to go with this.

Deadline added for the decision, 02.09.2019 end of work day.

@Lydia_Pintscher I added two entries to the AC for RDF URI output and for What Links Here functionality to be fixed. If that looks good on your end for releasing this feature, let's move it to Ready to Estimate.

TODO: Just wanted to highlight that once decisions are made... please ensure to update the Glossary item ! Currently it reads:

EntitySchema is a special type of Wikidata page containing a document in ShEx format, and related metadata. Although it may have labels, descriptions and aliases similar to items, it is not a type of entity, nor powered by Wikibase. Entities may be validated against an EntitySchema using a tool.

As a Data Architect in real life working with databases & entities, I actually appreciate and like the fact that EntitySchema is not a type of entity and as @alaa_wmde states, it decouples concepts and allows flexibility with multiple viewpoints from around the world. It also allows external publishers to express their own views and later link them and validate them. (not every entity/thing has to be stored in Wikibase, but allowing conceptual linking helps the world, so a canonical URI is a "good thing" and agree with @Jheald )

@Lydia_Pintscher I just discussed with @WMDE-leszek about how we should move forward with this. The ticket should probably be evaluated again, and then prioritize. Basically, nothing prevents us to move forward with linking Schemas in statements.

Is there anything inherently wrong or technically infeasible or undesirable, if an id used 2 letters? ES45 versus E45 ?

One suggestion made in Project chat https://www.wikidata.org/wiki/Wikidata:Project_chat#How_to_resume_a_property_proposal_being_on_hold was to bring attention to our requirement here.

We are currently trying to bring back this property discussion, currently 'under hold' : https://www.wikidata.org/wiki/Wikidata:Property_proposal/Shape_Expression_for_class. A number of entity schemas have been added during the BioHackathon. There are now around 200 Entity Schemas.

Reevaluation might be easier after having a dedicated triplestore available, please see T271065.

In T214884#5395432, @Lydia_Pintscher wrote:

Ok. Based on the comments here and in project chat let's flip the config switch for this to off and see what we can do about the raised points.

Project Chat can be found here: https://www.wikidata.org/wiki/Wikidata:Project_chat/Archive/2019/08#New_data_type_for_EntitySchemas