AIUI, this is a clone of some sort of the original RISC OS (the version that shipped with Acorn machines). It does feature some real improvements, such as being 32-bit clean which is needed for modern ARM hardware (the Acorn Archimedes had a 26-bit address space).
It's not a clone as much as a later version. After the demise of Acorn Computers development of RISC OS continued, but split (somewhat acrimoniously) between two different companies, both producing different versions from diverging descendants of the original source tree.
The RISC OS Open code is RISC OS 5, one of those two descendants. It's based on the original Acorn code, though obviously much improved over the last Acorn version. The most significant change is, as you say, that it uses the standard ARM 32 bit addressing mode, rather than the original 26 bit mode (support for which was removed from ARM chips in the mid 90s). Consequently it has been made to run on Raspberry Pi and various other modern SBCs.
There's BBC Basic in RISC OS Open; you can configure a Pi to start up at the Basic prompt like an old BBC if you like.
It's not the only implementation, though: the language had also been cloned as Brandy Basic (GPL) and also reimplemented in various commercial versions, notably BBC Basic for Windows. In times past there was also a compiled version (ABC) for RISC OS, though I don't know if it's still functional.
