Technologiehaufen sounds refreshingly honest if what you are referring to is the usual tangled mess of tools hotwired together from the repositories, and not the carefully vetted selection of neatly interlocking components people dream of when they say "tech stack". The latter might be called Basistechnologiekompisition which did not even exist in the Google index until now. If you insisted on making a direct translation, you should at least go with Stapel, not Haufen.

Tangent: stack (as in LIFO) is traditionally translated Keller instead of the much more literal Stapel, likely to avoid ambiguity between batch (also Stapel) and stack. One of the few ancient computer English translations that absolutely did make it into the 21st century is "Stapelverarbeitung". I think that this is because it sounds just as clumsy as the concept it is describing. When you say Stapelverarbeitung you can almost feel that stack of punched cards that you have never worked with if you are my generation.