This is not possible You need to map your old URLs on the news with referrals for SEO and user experience.
Google never forgets old URLs even after a decade. When you migrate to a new CMS, you must implement page-level redirects.
If there is no equivalent for a particular page, you can leave it 404 and Google will remove it from the index. Instead, using "deleted 410" allows Google to remove index URLs as soon as they are crawled without the 24-hour grace period used by Google for "404 no found".
There is no directive to tell robots not to use an old site in the search console or robots.txt file.
And if you do not redirect?
It may be that the redirection is too much work or that your new CMS does not make it easy to implement the redirection.
If you choose not to implement redirects, it will be something like starting over again. Google will see that your old URLs return the 404 status and remove them from the search index.
Your new URLs will eventually be indexed, but this may take some time. Changing any of your URLs without redirects is a serious sign that your site is not stable and can not be trusted. All your rankings will be lost and your site will start again.
Googlebot will continue to explore old URLs for years. For that, hope is eternal that you can one day go back these pages.
If you redirect, all your incoming links, user bookmarks, and most of your current rankings will be preserved.
So, why do search engines have no "reset" button? Because there are almost always better options. In your case, it is much better to redirect.
In the event that a site is penalized, Google does not provide a reset button, as this could remove all penalties.
So, how do you implement redirects? You need a list of your old URLs. You can start with a sitemap of your old site. You can also get the list from your server logs, Google Analytics, or even the Google Search console.
If you have planned in advance, your URLs in your new CMS will be similar and you can implement a rewrite rule to manage them. If there is a pattern between the old and the new URL, it can act from one line in one .htaccess file to send redirects for the entire site.
If you need to manually search for new URLs and map thousands of them one by one, you can consider: