Deprecation proposal: legacy Jinja template suffixes ('_t' vs '.jinja')

[jayaddison]

For a long time (since 2f18cc9 in Y2009, ~17 years ago), Sphinx has supported template evaluation of static files within projects. In particular, static files that have a _t suffix are evaluated as templates, and Jinja has always been the rendering engine used to evaluate them.

From #11916, first released with Sphinx v7.4.0 in July of Y2024 (~18 months ago), Sphinx has supported evaluation of template files when they have a .jinja filename extension, in addition to retaining the existing support for templates with the suffix _t.

Sphinx itself has migrated its internal templates to use the .jinja extension instead - but not all downstream projects, extensions and so on will have done yet.

Can we begin adding deprecation notices when files with the legacy _t suffix are evaluated?