Some of my diagrams includes another *.puml file (in my case ex. ../C4.puml)* and based on changes in the include file the generated images should be changed too (details see https://forum.plantuml.net/13923/how-can-i-clean-cached-include-files-on-plantuml-server). But the main diagram file itself is unchanged and therefore the cached imaged is returned.
(this can be simulated with simpler sudoku document too, details see https://forum.plantuml.net/13978/dynamic-content-changed-include-calculated-cached-for-days).
Therefore it would be handy that I can clean the outdated images from the cache via
'curl -X PURGE <imageUrl>',
eg. for the sudoku sample
'curl -X PURGE //www.plantuml.com/plantuml/png/9Smn3e8n24VnlQVuRnx0k7-tS4PAR2E5f42ylarStlJoko9fPorjDpnd3h2l9PO1SvXy4TNzKvW19cKjfPIE5yMf6-uFyA0D9XxJtks_jqki7_G7'
The Github's Camo cache supports same pattern, details see https://forum.plantuml.net/7163/githubs-aggressive-caching-prevent-diagrams-updated-markdown.
I would extend the pattern, that a purge deletes automatically all cached formats (png, svg, ...)