Hbs files continue to be “supported” (i.e. they’ll work, and we won’t change that without a deprecation cycle)
But yes, it’s strongly recommended to use .gjs for new code, and to start migrating existing themes & plugins. The most up-to-date linting config in the skeletons will enforce that recommendation unless you deliberately opt-out of the ‘require-strict-mode’ ember-template-lint rule.