The phrase "a key issue then" is correct and usable in written English.
It can be used to highlight an important topic or concern that was relevant at a specific time or in a particular context.
Example: "In the discussion about climate change, a key issue then was the impact of industrial pollution on air quality."
Alternatives: "a crucial matter at that time" or "an important point back then".
The key issue then is whether it is possible to draw a line between these debates and moral ones and to declare moral debates to lie outside of science proper.
The key issue then because whether it was acceptable to assume an upward-sloping short-run AS curve even though we had no "microfoundations" for that assumption, just observation of reality.
The key issue then becomes how to find the subgraph matches (of SPARQL query Q) that cross multiple fragments these are called crossing matches.
The key issue then is to better understand why some animals behave appropriately under novel conditions, while others do not.
The authors suggest that the key issue then becomes rapidly establishing the duration of ventricular fibrillation, in order to establish which patients might benefit from this approach.
The key issue, then, remains how do we maintain excellent breast conservation rates after lumpectomy, while reducing the morbidity of whole breast radiotherapy (WBRT)?
If the presynaptic α-synuclein aggregation is the key issue, then PD and DLB may eventually be treatable in future, because the neuronal cells and their presynapses are, in principle, still present and the postsynapse is able to regenerate.
A key issue early on, then, will be figuring out what court or courts will oversee the litigation and what law will apply.
A key issue for researchers then becomes one of accuracy; that is, whether the combination of identifying variables used is sufficiently precise to identify the correct person, but not so broad as to incorrectly match to another person who has the same demographic data.
