Open Safari Preferences window (by going to Safari Menu bar → Preferences... or using the keyboard shortcut Command + ,), go to the General tab and check if Remove download list items: is set to Upon successful download.
If yes, then the Download button is shown in the Safari toolbar (towards the right end of the Address and Search bar) as long as one or more download tasks are in progress. As soon as the last active download task completes, the Download button is automatically removed from the toolbar.
The Show Downloads command is also greyed out under the View menu if the Downloads button is not visible in the toolbar.
To view the button, you can change the Remove download list items: setting to a different desirable value.