If you want to disable the screenshot thumbnail preview in macOS:
• Go to the /Applications/Utilities/ folder and open the "Screenshot" application
• Click on the "Options" menu in the screenshot toolbar
• Uncheck the option for "Show Floating Thumbnail" to disable the screenshot preview
• Exit out of the Screenshot application
With "Show Floating Thumbnail" disabled, the screenshot previews will no longer appear, and the screenshot that has been snapped will be created and appear almost instantly in the Finder, much like in earlier macOS versions.