You'll be expected to support the same browsers as Bootstrap's core does. Always go to that link to confirm what exactly is supported and isn't but at a high-level here's an overview:
Bootstrap supports the latest, stable releases of all major browsers and platforms. On Windows, we support Internet Explorer 10-11 / Microsoft Edge. We do not officially support Internet Explorer 9 or lower.
For Firefox, in addition to the latest normal stable release, we also support the latest Extended Support Release (ESR) version of Firefox.
Generally speaking, Bootstrap supports the latest versions of each major platform’s default browsers. Note that proxy browsers (such as Opera Mini, Opera Mobile’s Turbo mode, UC Browser Mini, Amazon Silk) are not supported.