It's a real problem now in 2026. The accusation stings because it attacks your credibility for something you actually did. as some answers noted, some authors do post drafts or process documentation, but that's putting the burden on you to prove a negative, which sucks.
Realistically though, most serious readers and industry people won't care what a random Redditor thinks. Agents, publishers, and actual book critics judge on whether the work stands on its own merits. If your novel's good, it'll speak for itself. One thing that does help: if your book gets picked up by a publisher, they vet your manuscript thoroughly anyway. That vetting process basically solves the credibility problem because a publishing house isn't going to stake their reputation on AI-generated content.
The harder part is just living with the fact that "polished writing" now triggers suspicion in some people. That's the world we're in. Some authors lean into talking about their craft - interviews about revision, their influences, how long they sweated over specific scenes. Not because they have to defend themselves, but because the conversation about writing process is actually interesting to readers. But you don't owe anyone that either. You wrote it. You know you wrote it. That has to be enough.