My team recently evaluated 30-40 CSS frameworks to use with a custom component library, and we were surprised when Bootstrap was the clear winner. Other CSS frameworks have gotten buzz over the years, and we assumed something new and shiny would come out on top, but when we evaluated all of them, we saw sporadic commits, many unresolved issues (e.g., see Bulma issue 3074), small bus factors, inadequate support for keyboard navigation, and other issues.

So using Bootstrap with a newer alternative to React is valid. (Though, we're using Bootstrap with Next.js.)