It can be tricky. Everything does go by UY of the points being transferred. So, as you mention, someone who is in their 2021 UY, can only get 2021 or 2022 UY points right now.
However, anyone still in their 2020 UY can get 2021 UY points that can actually be used in their 2020 UY as long as the trip happens after the 2021 UY points are valid.
For example, I transferred June 2020 UY points into my Dec 2019 UY. I was able to use them for trips during both my 2019 and 2020 UY, since transferred points retain their own usage rules.