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In the early days of when Google first introduced XML Sitemaps, Google also a collection of 3rd party sitemap generation solutions. That page now shows a notice that the top that reads "this Google Code project has been deprecated and is no longer maintained."
Why? Well, most CMS solutions have an XML Sitemap solution that is natively implemented within it. Or there are several supported plugins for the CMS tool for XML sitemap generation.
John Mueller of Google said on Twitter "most CMS's have good plugins or native implementation nowadays," when responding about that page that is deprecated. "Most don't need 3rd-party tools," he added.
That's been stale for a really long time - most CMS's have good plugins or native implementation nowadays. Most don't need 3rd-party tools.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) June 11, 2017
And if you do go custom for a CMS then the developer is likely to build into it a native XML Sitemap feature. Like this site. :)
Forum discussion at Twitter.
