What are the impacts of closing STEM departments?
I can't give you a 100% answer (your question is kind of too wide) but I will try to give you some ideas:


The UK evaluates all departments of universities and research institutes on a
regular basis by a so-called Research Excellence Framework (REF). A bad REF evaluation might force a university to shut down a department. This is because universities are only allowed to teach a subject if they also do well received research in this field. Similar evaluation evaluations might be in place in other countries.
It might be a financial issue of why a department shuts down. Too few students and too high costs (in STEM there are often not enough good students applying)
"without the usual qualifications in Mathematics" I would not see this necessarily as a negative point. If you allow people with different backgrounds into a degree you might create a cohort of interdisciplinary graduates that might be in demand in industry. (They might not be competitive with e.g. pure math graduates but how many people really work in pure math later?) But it might also be due to the previous point that not enough people apply with a strong enough math background.
One more possible reason is that the university is not able to hire/hold sufficient professors in STEM fields as the pay in industry is usually much better and people with strong technical background might be aggressively recruited away. 


Impacts:


It is difficult so say what the impacts are as they are university specific but some degrees will not be offered anymore and/or they get external lecturers for e.g. math courses in non-STEM-degrees.
On a whole population level: Fewer people with STEM degrees means
fewer qualified workers
On the positive side: The university might have free-up resources in
order to persue other fields more intensively - but that is more a
strategic decision.