"Not yet" means you ship with what you have, then you fix it. "Never" means accessibility was never in the plan to begin with.
The difference is in the intent, not in the code.
A product team that says "not yet" has a ticket open, there's a timeline forming and there's maybe even a person assigned. The team that says "never" just calls it out of scope and moves on.
But then there's a third team.
The silent never. They never made any decision because they never had the conversation. Accessibility just didn't exist for them. At least the team that says "never" considered accessibility and passed on it, for whatever reason. But the team that silently said never, never considers it to even know what they're saying no to.
The not-yet teams ship better products over time. The never and silent-never teams just ship the same broken experience, version after version.
Not yet is different from never.