The answer is mostly no. The “enhanced” text widget was designed to work like the post editor as much as possible, including autop which might break whatever HTML that can not stand the conversion of lines into paragraphs.
Several people have released plugins to restore the former functionality, but 4.8.1 will also include an “code” widget that can be used to add unaltered HTML. The problem is that there is no, and unlikely to be a plain upgrade path. Best advice right now if you use the html widget is to skip 4.8 and wait for 4.8.1 and allocate some time in advance to migrate the widgets.
If you are just looking to hide the “visual” tab, you are most like also out of luck best to open a ticket at trac to add such possibility, specifically for the widget without impact on the post editor (4.8.1 plan right now is to use the same setting for both)