@strapi/plugin-documentation middleware which expose openapi.yaml & swagger.json. Tested with 4.x

strapi-plugin-documentation-extend-middleware.ts

/**
 * `documentation` middleware
 */

import { Strapi } from '@strapi/strapi';
import * as fs from 'fs';
import * as path from 'path';
import yaml from 'js-yaml';

function getDocumentationService(ctx, strapi: Strapi) {
  return strapi?.plugins['documentation']?.services['documentation'];
}

function getDocumentationVersion(ctx, strapi: Strapi) {
  const requestUrl: string = ctx?.request?.url;
  const paramVersion = requestUrl?.split('/')?.find(segment => /v\d+\.\d+\.\d+/g.test(segment))?.replace(/^v/, '');
  const docVersion = getDocumentationService(ctx, strapi)?.getDocumentationVersion()
  return paramVersion || docVersion;
}

function getDocumentationPath(ctx, strapi: Strapi) {
  return path.join(getDocumentationService(ctx, strapi)?.getFullDocumentationPath(), getDocumentationVersion(ctx, strapi), 'full_documentation.json')
}

export default (config, { strapi }: { strapi: Strapi }) => {
  return async (ctx, next)=> {
    const requestUrl: string = ctx?.request?.url;
    const requestRegExp = /^\/documentation\/(v\d+\.\d+\.\d+\/)?(openapi\.yaml|openapi\.yml|swagger\.json)$/;
    if (requestRegExp.test(requestUrl) && ctx.request.method === 'GET') {
      const documentationService = getDocumentationService(ctx, strapi)
      if (!documentationService) {
        ctx.throw(500, 'Documentation service not found');
      }
      try {
        const documentationVersion = getDocumentationVersion(ctx, strapi);
        const documentationPath = getDocumentationPath(ctx, strapi);
        if (!fs.existsSync(documentationPath)) {
          ctx.throw(404, `Documentation not found for version ${documentationVersion}`);
        }
        const jsonContent = fs.readFileSync(documentationPath, 'utf8');
        const jsonData = JSON.parse(jsonContent);
        const isJson = requestUrl?.endsWith('.json');
        const isYaml = requestUrl?.endsWith('.yaml') || requestUrl?.endsWith('.yml');
        let body = jsonData;
        if (isYaml) {
          body = yaml.dump(jsonData);
        }
        if (isJson || isYaml) {
          ctx.body = body;
        }
      } catch (e) {
        ctx.throw(500, e);
      }
    }
    await next();
  };
};