How to prevent applications from uncovering the technologies used on the website Here is a super hard question, probably for the most experienced people: 

Can we prevent applications from uncovering the technologies we are using.

We all know that making sure to keep the core and contributed modules up to date is one of the most important aspect of web development. But meanwhile hiding information about the used technologies will another layer of the security.

Looking in the https://drupal.stackexchange.com/ and Google I can see the last time people talked about this topic was 2013, I think this is the time to ask this question again with hope that someone now have a solution for this.
Making all the Drupalisms disappear is hard work and most definitely not worth your while, as stated above by Clive. In addition, you'd also have to edit/obfuscate many of your http headers. 

At that point, if it were me visiting your site all the SSL and Trustee certs could not make up for trust lost and I'd leave. Plus, I might tell my nerd friends what a suspicious weird site you run and turn them off too. 

My advice: Don't break what makes the web work, and secure your site. You can and should stop nginx and PHP from advertising their version numbers, and you should delete Drupal's .txt files.