The sources explain the significance of Microsoft Bing's search index for SEO due to ChatGPT Search using Bing's data to power its own search engine. The sources recommend that SEO managers audit their sites to ensure they are being indexed by Bing, as being absent from Bing's index means being absent from ChatGPT Search results as well. The authors emphasize the importance of proactive SEO practices, suggesting that a quick check-up of Bing indexing can be an easy and valuable step towards future-proofing SEO strategies. Finally, the sources argue that Bing's role in ChatGPT Search is likely to remain significant in the foreseeable future, making it crucial for SEO managers to prioritize Bing indexing for better visibility.