I have recently discovered that my ~/.cache/thumbnails/normal/ folder contains some very old thumbnails. I was under the impression that these were automatically deleted with reasonable frequency. The contents of this folder suggests that I am wrong. I had a similar issue in Ubuntu 18.04 with GNOME and was under the impression that this was a Nautilus issue. However, having this same issue on Xubuntu 20.04 with Thunar suggests that I need to look elsewhere.
Are the deletion rules for the files in ~/.cache/thumbnails/ somewhere that I can easily access and change? Furthermore, is this an Ubuntu issue or have I simply had the misfortune of using two DEs with an identical issue?