TypeError: context.getScope is not a function — ESLint 9 moved rule-context methods to sourceCode

1. Upgrade the plugin to its latest major.
2. If it is still not updated, wrap it with @eslint/compat:

// eslint.config.js
import { defineConfig } from 'eslint/config'
import { fixupPluginRules, fixupConfigRules } from '@eslint/compat'
import somePlugin from 'eslint-plugin-some-plugin'
import someConfig from 'eslint-config-some-config'

export default defineConfig([
  ...fixupConfigRules(someConfig),
  { plugins: { somePlugin: fixupPluginRules(somePlugin) },
    rules: { 'somePlugin/rule-name': 'error' } },
])

3. For your own rules, migrate the calls:

// before                         // after
context.getScope()                sourceCode.getScope(node)
context.getAncestors()            sourceCode.getAncestors(node)
context.markVariableAsUsed(name)  sourceCode.markVariableAsUsed(name, node)
context.getSource()               sourceCode.getText()
context.getSourceCode()           context.sourceCode
context.getFilename()             context.filename
context.getCwd()                  context.cwd
context.parserOptions             context.languageOptions.parserOptions

(where const sourceCode = context.sourceCode). Automate with npx codemod @eslint/v9-to-v10 or eslint-transforms v9-rule-migration rules/.