To the best of my knowledge the HAL was added by PACE for set top boxes, and later removed by RISC OS Ltd. for the RISC OS Select versions, though the Castle versions retained the HAL, despite the solution providedin RISC Select being more in line with the way that RISC OS works.
I am not sure of the about the HAL being " later removed by RISC OS
Ltd. for the RISC OS
Select versions", in that RISC OS
4 (which predates Select) didn't have a HAL either and was produced by RISC OS
Ltd. As far as I know (and I am open to correction here) ROL didn't use a HAL because they hadn't got one - and given that the hardware they were developing for had the required Acorn designed parts (IOMD/VIDC) it would have (arguably) not made sense to include a HAL even *if* they'd had one.
Version 5 (The Castle/Tematic one was probably - as you say derived - from work for PACE) it did include a HAL in that Castle needed that to run RISC OS
on what amounted to be "alien" hardware for their Tungsten/Iyonix machine. To do that in a timely fashion needed a HAL (remember RISC OS
was built to run on an IOMD/VIDC equipped machine - and Iyonix contained neither and had a 32 bit (only) ARM, PCI and a NVidia video card to deal with as well).
The fact a 32 bit hardware abstracted version of RISC OS
made it onto "non-traditional" hardware *first* is vindication of the approach. Having a working RISC OS
that does use a HAL it would not make sense to (IMHO) for it to then be removed as it would break the OS
and require a major re-write for (arguably) little benefit.