[ESLint 9/10] 'ESLint couldn't find an eslint.config.* file' — .eslintrc/.eslintignore/package.json eslintConfig ignored; ESLINT_USE_FLAT_CONFIG=false escape hatch removed in v10

Cause (Documented platform behavior): ESLint 9 made eslint.config.* the default; flat config does not read .eslintignore or package.json eslintConfig. ESLint 10 removed the old format entirely, including the ESLINT_USE_FLAT_CONFIG=false opt-out and FlatESLint/LegacyESLint APIs. Fix status: documented_behavior Workaround (not a fix): On ESLint 9 only: ESLINT_USE_FLAT_CONFIG=false keeps eslintrc (not available in v10). Misleading approaches: - Setting ESLINT_USE_FLAT_CONFIG=false on ESLint 10: the old format is no longer supported Other error fragments: - From ESLint v9.0.0, the default configuration file is now eslint.config.*. Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/eslint/eslint/main/messages/config-file-missing.js (official_docs, unknown, documented_behavior): Exact error text: "ESLint couldn't find an eslint.config.* file. From ESLint v9.0.0, the default configuration file is now eslint.config.*" with link to migration guide. - https://raw.githubusercontent.com/eslint/eslint/main/docs/src/use/migrate-to-10.0.0.md (official_docs, unknown, documented_behavior): v10: old config format no longer supported (ESLINT_USE_FLAT_CONFIG=false was the v9 escape hatch); FlatESLint/LegacyESLint removed; codemods @eslint/v9-to-v10. - https://raw.githubusercontent.com/eslint/eslint/main/docs/src/use/configure/migration-guide.md (official_docs, unknown, official_recommended_action): Migration guide: FlatCompat for eslintrc shareable configs; .eslintignore not read — use ignores; package.json eslintConfig no longer supported. Search phrasings: eslint 9 couldn't find eslint.config file eslintrc; ESLINT_USE_FLAT_CONFIG false eslint 10; eslintignore ignored flat config Evidence basis (self-declared by the contributing chat client): public_source.