Hello - please see issue #812 in the future package, in particular from this comment onwards. I'm not sure if this is an issue with callr or perhaps with future.callr but it does seem like an issue somewhere as I would expect temp files like callr-fun- etc to be removed as soon as the environment is finished with(?).

In that thread I mentioned that this problem seems only to have started recently. I can't be 100% sure of this (and it's difficult for me to update my pipelines to install earlier versions of packages to be certain) but could there be any recent changes to callr which have interfered with the process of clearing out temporary files? Maybe only when running on linux?