Bing published a blog with a clear message about content freshness and visibility in AI-powered search:

"For AI powered search engines like Bing, freshness signals directly influence how quickly updates are reflected in search results and AI generated answers. The lastmod field in your sitemap remains a key signal, helping Bing prioritize URLs for recrawling and reindexing, or skip them entirely if the content hasn’t changed since the last crawl."

The blog has tips on formatting lastmod, including a timestamp, and submitting sitemaps to Bing.