Update Starting from biblatex 3.13 a rudimentary version of labelprefix is supported with BibTeX. Details can be found at https://github.com/plk/biblatex/issues/852. It is still strongly advised to use Biber, though.
At the moment the BibTeX backend only supports one refcontext for the entire document. In particular that means that you can't have several labelprefixes with BibTeX any more. This is one of the things biblatex warns you about when you run BibTeX
Package biblatex Warning: Using fall-back BibTeX(8) backend:
(biblatex) functionality may be reduced/unavailable.
I'll have a look whether there is a way to get back at least a bit of labelprefix with backend=bibtex, but I can't raise your hopes, the whole system is quite a bit more complicated than what simple BibTeX usually allows and BibTeX development is not a priority. (See https://github.com/plk/biblatex/issues/852 - not sure if this will lead anywhere.)
As for your issue of getting labelprefix to work for a submission to the Software Quality Journal I don't think using biblatex is going to be a good idea. Most publishers don't accept biblatex submissions (Biblatex: submitting to a journal) and I don't think Springer is an exception. I couldn't find a link to a LaTeX template in the Author Guidelines for that journal https://www.springer.com/computer/swe/journal/11219?detailsPage=pltci_2530611, but the normal Springer templates use BibTeX-based bibliography solutions and not biblatex. Unless a publisher explicitly states they want biblatex I would always assume that they either want their house .bst file, a simple .bst like plain or a manual thebibliography. The biblatex workflow is very different from the normal bibliography workflow. For the most part the fundamental problems that publishers have with accepting biblatex submissions occur regardless whether Biber or BibTeX is used.
The citation style required for the Software Quality Journal (a simple author-year system) does not really lend itself to labelprefix (which is usually only used with and in standard biblatex only supported by numeric or alphabetic styles), so I doubt that you should use it at all.