Currently if you create an EntitySchema there is no default/example validator to use to check it against items, this can make users unfamiliar with Shape Expressions be confused on what it is meant for and over all make it harder for people to utilize these schemas. Having a linked ShEx validator which can be done in the default wikibase install: https://www.mediawiki.org/wiki/Extension:EntitySchema and is currently used by wikidata https://www.wikidata.org/wiki/EntitySchema:E192 would be a great quality of life feature and make schemas easier to use.

$wgEntitySchemaShExSimpleUrl needs to be set to a URL of a ShEx validator, the one suggested on the documentation page and appears to be used by wikidata is "https://tools.wmflabs.org/shex-simple/wikidata/doc/shex-simple.html"

example of what the resulting URL on a EntitySchema page should look like if done successfully:

https://shex-simple.toolforge.org/wikidata/packages/shex-webapp/doc/shex-simple.html?data=Endpoint:%20https://mcje-data.wikibase.cloud/query/sparql&hideData&manifest=[]&textMapIsSparqlQuery&schemaURL=//mcje-data.wikibase.cloud%2Fwiki%2FSpecial%3AEntitySchemaText%2FE3