Well, if you want to build something that has animations and feels like a native app (e.g. PWA), you will probably skip bootstrap (and take something like framework7 [1]). But if you just want to build a plain old server rendered website bootstrap is still very useful as a reliable default theme for your HTML documents.

Nevertheless, recently I tried spectre.css [2] and liked it a lot better than bootstrap as it doesn't feel so heavy (don't know how it performs in terms of browser compatibility).

[1]: https://framework7.io

[2]: https://picturepan2.github.io/spectre/