I may be misunderstanding, but if you’re asking if it’s possible to make a toggle that hides the top brown bar in Webflow, the answer would be yes, it is possible and likely very easy.
However, if you’re asking whether Webflow can have a user account system with registration and login (based on what you highlighted in the photo), then no, it cannot. Webflow is primarily and most powerfully a front end web design tool, and so anything pertaining to front end development is possible in Webflow (especially if you’re a creative problem solver). It also has a very complex and powerful CMS system. New features are constantly being worked on and released, like site search and e-commerce, so I don’t think user registration is off the table in the future. I encourage you to visit the Webflow wish list to be up to date on coming features.
Hope that helps!