Arrows shot by skeletons are not despawning. They are counted as entities (unsure if that's intentional or not) but they do not despawn. Removing the block they are attached to sometimes makes them disappear, but they are still counted as an entity, ghost entity if you will. This causes a lot of lag in servers, especially ones with multiple skeleton grinders.
Linked issues
Comments 2
Duplicate of MC-MC-31577 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.
Cannot Reproduce. They despawn as normal (each one after 1 minute or so).