Average shelf life still looks to be 24 months. There are a decent number of sets each year that get the 18 month treatment, and a good amount that get 12 months or less. Still some are extended well beyond 24 months based on sales.
I wouldn't call it a downward trend at this point. You have a correction after 2020 where they produced a ton to meet new demand. And now they've moved more dollars towards the high end adult market. The consumer dollars are spent on fewer sets at a higher cost, which still takes from availability to spend on the secondary market.
About 75% of sets get that 24 month treatment.
Nearly 100 sets released in 2022 got 12 months or less. 2023 currently has 79 sets on tap to retired in 12 months or less. But most of these usually are small sets, polybags, Mario, and other various turds.