misc: Remove migration UI, warnings, and errors for changes pre-Cypress 10 (v15)

[jennifer-shehane]

• Closes Remove migrations scripts for v9 -> v10 upgrades #31345

Additional details

🎉 13,000+ lines of code removed! 🎉

• Technically, I didn't think this should be considered as 'breaking', it won't break users - there are just some situations that will behave differently in 15 vs 14 when using removed functionality. I've flagged it as 'misc', but open to feedback.
• Migration UI, warnings and related errors are no longer shown when upgrading from Cypress versions earlier than 10.0.0.
• Projects with cypress.json files will not be suggested to migrate.
• Errors and warnings will no longer display for users using configuration values removed in versions before 10.0.0. Including:
  • blacklistHosts
  • componentFolder
  • experimentalComponentTesting
  • experimentalGetCookiesSameSite
  • experimentalNetworkStubbing
  • experimentalRunEvents
  • experimentalSessionSupport
  • experimentalShadowDomSupport
  • firefoxGcInterval
  • ignoreTestFiles
  • integrationFolder
  • pluginsFile
  • testFiles
• Users using the cypress-plugin-retries package will no longer receive an error to switch to 'retries' configuration.
• Users passing matchUrlAgainstPath to cy.intercept() will no longer receive an error (removed in 7.0)
• Users passing --canary will no longer see an error (removed in 4.0)
• Users registering dev-server:start listeners will no longer receive an error (removed in 10.0)
• Mention of changes made in Cypress 10.0 were removed from error messages - this is just standard behavior.
• Removed Cypress 10.0 from the major version welcome screen (summary of major versions) which was released 3 yrs ago.
• Many of our tests and snapshots were updated to be more accurate - removing old uses of cypress.json in our tests for example.
• Removed an ENORMOUS amount of system tests that are no longer relevant.

Steps to test

• Ensure tests pass
• We want to especially ensure logic around loading config and showing invalid config errors is working as much of this was touched.
• Also ensure the Welcome screen shows for a major version update

How has the user experience changed?

Migration helpers and related errors are no longer shown when upgrading from Cypress versions earlier than 10.0.0.

PR Tasks

• Have tests been added/updated?
• Has a PR for user-facing changes been opened in cypress-documentation?
• Have API changes been updated in the type definitions?

[AtofStryker]

binaries to test are here

[Copilot]

Pull Request Overview

This PR removes legacy migration UI, warnings, and errors for pre‑Cypress 10 configurations while also cleaning up related code and tests. Key changes include:

• Deleting the entire MigrationDataSource and LegacyPluginsIpc modules.
• Removing migration-related logic from data sources, actions, and the project lifecycle manager.
• Updating tests, snapshots, docs, and CI configurations to reflect these removals.

Reviewed Changes

Copilot reviewed 457 out of 457 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/data-context/src/sources/MigrationDataSource.ts	Removed legacy migration data source and related migration functions.
packages/data-context/src/data/index.ts	Removed export for LegacyPluginsIpc to clean up unused code.
packages/data-context/src/data/coreDataShape.ts	Cleaned out migration interfaces and state from core data structure.
packages/data-context/src/data/ProjectLifecycleManager.ts	Removed legacy migration checks and methods; adjusted conditions for configuration processing.
packages/data-context/src/codegen/spec-options.ts	Updated import path to use the new util folder instead of migration utilities.
packages/data-context/src/actions/*	Removed migration actions from both index and ProjectActions modules.
packages/config/*	Updated tests, config options, and snapshots to remove references to deprecated migration options and legacy warnings.
npm/grep/README.md, guides/error-handling.md	Updated documentation to refer to the new config file format and error messages.
.circleci/workflows.yml	Updated branch filters to reflect the removal of migration-related branches.

Comments suppressed due to low confidence (5)

packages/data-context/src/sources/MigrationDataSource.ts:1

• Ensure that all references and imports of MigrationDataSource are removed from the codebase so that no broken dependencies remain.

/* Entire file removed */

.circleci/workflows.yml:41

• The branch filter was updated to 'remove-migration'; please ensure that this change is coordinated with all related CI configurations to avoid unintended build exclusions.

-        - 'remove-migration'

packages/data-context/src/data/ProjectLifecycleManager.ts:761

• [nitpick] The removal of the migration check in the e2e condition simplifies the logic; please confirm that this change is intended and that no legacy behavior is required for e2e projects.

if (testingType === 'e2e') {

guides/error-handling.md:81

• [nitpick] The error message now references 'fakeFile' instead of 'pluginsFile'. Confirm that this change in terminology is intentional and that the documentation reflects the new naming convention.

FAKE_ERROR: (arg1: string, arg2: Error) => {

packages/data-context/src/codegen/spec-options.ts:6

• Verify that updating the import path for getDefaultSpecFileName aligns with the current project structure and that there are no residual dependencies on the old migration utilities path.

import { FileExtension, getDefaultSpecFileName } from '../util/files'

[AtofStryker]

not sure I understand the skip here? We should probably create a git issue and explain this in more detail and link it back here

[jennifer-shehane]

Currently we don't have any renamed config option errors that are active. So, we haven't renamed any configurations since 10.x (where there would be a key of newName in the breakingOptions), so I don't have a real use case to test here. There's already a unit test for this behavior, this was checking integration.