Most website nowadays require Javascript in some capacity to function. Javascript is a technology that allows small programs to run on a webpage to make your experience better or to facilitate necessary functions on the site. It has been around since 1995 and has cemented itself as a staple of the web as we know it today.
While some websites do allow you to use them without Javascript enabled, this is more and more rare as Javascript has become so engrained in the usage of websites.
Windscribe is no exception to this, there are necessary functions that Javascript handles on our website which is why it's required when trying to sign up.