I will post a controversial answer with which many will probably disagree. Is Sitemap.xml still necessary? This is no more crucial, no. Is it good to have and help? Sure. Do you have to delete it? I can not say, because I do not know what are the reasons above.
Take a look at this article, written by my former colleague with several years of experience in the areas of SEO, user experience and web development; I agree with a lot of this:
If you do not actively update your sitemap and do not send it to GSC or Bing Webmaster, you probably do not need it. A sitemap is a suggestion to search engines on how your site should be explored. Think of sites like news sites with hundreds of thousands of pages: sitemaps have size limits. They do not contain all published articles, but these articles are found, indexed and classified.
If your site is updated regularly; has a clear information architecture and a hierarchy of categories; and includes a well-planned internal link structure, the search engines will eventually index and classify the content, regardless of the existence of the sitemap.
However, keep in mind that you will lose the ability to submit the Sitemap manually in GSC, which triggers an analysis again. You will also lose the ability to suggest to search engines which parts of your site are the most important or what are the latest articles.
Many SEOs still say that XML sitemaps are crucial, but that is the conventional wisdom of SEO. Crucial is not how I would define it; Helpful is more like that, from what I've observed in recent years. Regarding your case, it will depend on the reasons for which you delete it and the duration of your maintenance.