Essentially, good quality writing is achieved by clearly stating your main points and supporting those within the analysis that follows, which would typically involve substantial reference to techniques and quotes within your prescribed texts that would be most appropriate in terms of supporting your main points and analysis. Adopting a particular paragraph structure/format subsequently allows you to develop your paragraphs and create a logical sequence that would suit the question that you are responding to.
Given the purpose of an introduction and a conclusion, it is evident that those would need to be separated from body paragraphs, which would be the ones where paragraph structures such as STEEL and STEAL are most relevant. Based on this, I fail to see a reason for deducting marks in the event that the introduction and the conclusion are separated from body paragraphs adopting STEEL/STEAL paragraph structures. Keep in mind that, with respect to essay writing, the emphasis is on the quality of your points and analysis, in addition to the links that you establish with the question and the module that it targets. Therefore, paragraph structures such as STEEL and STEAL do not constitute an assessable component.
I hope this helps!