Regardless of the right click. Anyone could turn on the dev tools in Chrome or Safari and inspect that way. If you upgrade to a pro plan there is still going to be a line included on Webflow hosted sites "meta name="generator" content="Webflow"" plus some other indicators in the CSS and Java Script I think.
I'd like to see the "generator" metaname come out of the Webflow code but I wonder if it's there for tracking. Someone in another thread suggested that the forms wouldn't work if that tag wasn't included...not sure that's correct but maybe. Maybe @cyberdave could confirm if that's the case.
However, Webflow is a professional tool. Designers and developers use all kinds of tools in their toolkits. Think Bootstrap, there are tons of sites built off that framework, some more customized than others.
I find that many/most clients could care less about the tools used. However, 100% of them are looking for a site that either helps establish their brand or strengths it, that looks good, that is mobile responsive, and that "comes up in Google". Designers use tools. Developers use tools. To be competitive, make an income and provide clients quality sites that meet their needs and expectations is really what it's all about!
Be transparent with clients, they will appreciate it. Maybe @sabanna, @Waldo_Broodryk or @vincent can weigh in with their opinions.