Ok, mal posso esperar!. Tentei com graphql-mesh.
Então, o arquivo OpenAPI disponível sob o botão de download em https://docs.discourse.org/ parece estar quebrado 
Testei a validade aqui com esta saída:
Swagger schema validation failed.
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/3
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/3
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/3
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/2
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/2
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/2
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/1
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/1
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/1
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/0
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/0
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/0
JSON_OBJECT_VALIDATION_FAILED
Error: Swagger schema validation failed.
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/3
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/3
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/3
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/2
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/2
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/2
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/1
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/1
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/1
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/0
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/0
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/0
JSON_OBJECT_VALIDATION_FAILED
at o (https://apitools.dev/swagger-parser/online/js/bundle.min.js:1:73766)
at https://apitools.dev/swagger-parser/online/js/bundle.min.js:17:227596
SyntaxError: Swagger schema validation failed.
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/3
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/3
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/3
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/2
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/2
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/2
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/1
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/1
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/1
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/0
Data does not match any schemas from 'oneOf' at #/paths//page_view_total_reqs/get/parameters/0
Missing required property: schema at #/
Missing required property: content at #/
Missing required property: $ref at #/paths//page_view_total_reqs/get/parameters/0
JSON_OBJECT_VALIDATION_FAILED
at Function.o [as syntax] (https://apitools.dev/swagger-parser/online/js/bundle.min.js:1:73766)
at validateSchema (https://apitools.dev/swagger-parser/online/js/bundle.min.js:1:5021)
at SwaggerParser.validate (https://apitools.dev/swagger-parser/online/js/bundle.min.js:1:3171)
z-schema validation error: JSON_OBJECT_VALIDATION_FAILED
at ZSchema.getLastError (https://apitools.dev/swagger-parser/online/js/bundle.min.js:17:211187)
at validateSchema (https://apitools.dev/swagger-parser/online/js/bundle.min.js:1:4925)
at SwaggerParser.validate (https://apitools.dev/swagger-parser/online/js/bundle.min.js:1:3171)
Então, procurei por outro arquivo OpenAPI ou Swagger da API do Discourse. Encontrei apenas este.
Ele está um pouco quebrado na linha 426, mas eu corrigi.
Depois de configurar o graphql-mesh, obtive uma API GraphQL funcional! Mas…
Este arquivo Swagger não está completo ou está muito antigo. Há apenas 4 consultas, sem mutações, o tipo user com poucos propriedades…
Existe em algum lugar um arquivo Swagger ou OpenAPI do Discourse?