Before Flex became popular (around 2014), the float-based grid system was the most reliable because it still has the most browser support among the alternative existing systems (flex, grid).
Bootstrap was using the float approach until Bootstrap 4 which switched to the flex-based approach. As of writing (2020), flex is the recommended approach for building grid systems and has decent browser support.
For the adventurous, they can look into CSS Grid Layout, which uses the shiny new grid property; it is even better than flex for building grid layouts and will be the de facto way to do so in the future.