Yes. It's fine for progressive enhancement of server-side-rendered webpages.
I think it is still widely adopted in CMSs like Drupal and Wordpress and they have a big market share. Jquery will not die soon even if it is not the first choice for starting a new project. It's been years since the last time I used it.
Absolutely. Starting most new projects with something like React/Vue is complete overkill and jquery makes adding small incremental JS functionality to your app a breeze.
I can't speak for React, but I found adding VueJS was perfect for adding JS functionality incrementally to a section of a site. See my other comment on this thread on using it to create a dynamic contact form to an otherwise static site.
I've had to learn it at a new job after starting my career making UIs with React. It's not so bad.