The idea is that if a machine defaults to "legacy boot," meaning a BIOS-style boot, then use that to load U-Boot, which then provides a software emulation of UEFI so that the startup process can be simplified by the removal of BIOS support.
At my company, we have around 400,000 servers in production. When we last surveyed them, we found several thousand over 12 years old, with the oldest at 17 years. And that wasn’t counting our lab and admin servers which could run even older because they’re often repurposed from prod decomms.
We had a huge internal effort to virtualize their loads, but in the end, only about 15% were transferred just due to the sheer number of hidden edge cases that kept turning up.
2014 is when a majority of new systems were UEFI, according to Wikipedia, but that's still a majority.
Intel announced in 2017 that by 2020 they're no longer gonna include BIOS support in their computers. So it could easily still pop up today, although it's not that likely to, since that support is for devices that can use either BIOS or UEFI.