Deprecate Loose Mode and hbs authoring format

[Copilot]

Deprecate Loose Mode and hbs authoring format

⚠️ Placeholder RFC generated with copilot, probably don't bother reading yet.

Rendered

Summary

This pull request is proposing a new RFC.

Formalizes the deprecation of loose mode Handlebars templates and .hbs file authoring as the logical conclusion of RFC #496 (Strict Mode), RFC #779 (First-Class Component Templates), and RFC #1046 (Template Tag in Routes) all reaching recommended/released status. Deprecates the loose mode compilation path, co-located .hbs component templates, and .hbs route templates under app/templates/ in favor of .gjs/.gts template tag files.

Scope of deprecation:

• Loose mode compilation/runtime resolution (implicit globals, implicit this fallback, dynamic resolution)
• Co-located .hbs + .js/.ts component pairs → .gjs/.gts
• Template-only .hbs components → .gjs
• Route-level .hbs templates under app/templates/ → .gjs/.gts

Not in scope: inline hbs tagged template literals in tests, the Glimmer VM itself.

Transition path: ember-codemod-template-tag for automated migration; new no-loose-mode-templates ember-template-lint rule; blueprint updates to generate .gjs/.gts by default; deprecation ID ember.loose-mode-templates.

To succeed, it will need to pass into the Exploring Stage, followed by the Accepted Stage.

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Upon merging this PR, automation will open a draft PR for this RFC to move to the Ready for Released Stage.

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Checklist to move to Exploring

• The team believes the concepts described in the RFC should be pursued.
• The label S-Proposed is removed from the PR and the label S-Exploring is added.
• The Ember team is willing to work on the proposal to get it to Accepted

Checklist to move to Accepted

• This PR has had the Final Comment Period label has been added to start the FCP
• The RFC is announced in #news-and-announcements in the Ember Discord.
• The RFC has complete prose, is well-specified and ready for implementation.
  • All sections of the RFC are filled out.
  • Any unanswered questions are outlined and expected to be answered before Ready for Release.
  • "How we teach this?" is sufficiently filled out.
• The RFC has a champion within one of the relevant teams.
• The RFC has consensus after the FCP period.