In October 2021, I had the same impression as the OP, that more questions were going unanswered lately, so I ran some up-to-date statistics.
We currently have 1,587 unanswered questions out of 7,436 (78.66% answered).
The rate at which unanswered questions are accumulating over time is non-linear. I overlaid a linear trend line for comparison.
This is based on a query in Data Explorer of all unanswered questions ordered by creation date.
Our answer ratio is thus still hovering around 80%, but there is an acceleration noticeable in the number of questions that go unanswered.
However, this acceleration seems to have mostly started late 2017, early 2018. For example, the unanswered question growth does look linear in the time range of 2019-2021.
Is it a coincidence this coincides with our site reboot initiatives? To thus answer the OPs concluding question:
Is it a requirement for citations and research that prevents there being so many answers as some of the other SE sites?
I expect so. Given that we made the contribution requirements more stringent around that time, it is a reasonable assumption that this has made it harder to answer questions on this site, explaining the increase in the rate at which unanswered questions accumulate.
But, luckily the data also shows the accumulation rate is no longer accelerating as drastically, although a slight uptick mid 2020 is noticeable.