With Vue, and other frontend libraries, do you still use the newest twitter bootstrap or is it obsolete?
I'd say... Use whatever you're comfortable with unless you want another problem to solve.
Nowadays the cool kids have all moved towards tailwindcss. It feels to me like a successor to bootstrap... It's bootstrap that only exports what it needs of itself. The problem is the disgusting html semantics that comes with it. How much do you have to hate CSS to duplicate weird class names, which are in essence a shadow of CSS within the code?... Over and over. It's like the 90s all over again and putting all the style within the html tags!
So I'd say, use bootstrap if you're comfortable with, it's fine, or roll your own.
I've used bulma[1] with some success for a small project, the problem is I always end up fighting the framework. It's hard to find one that doesn't limit you in some way, or pollute the namespace with a ton of junk you'll never use. On the other side of the stack you have tailwind which I'm becoming allergic to.
It's worth learning CSS, there is nothing hard. Most of my products/site are custom CSS. It's easy to jump back in and inspect old code. CSS stays CSS. Those frameworks come and go like the resume piece they started as.