Picking anything but PlantUML's default theme will make PlantUML look less 90's-Java. As a minimalistic solution, I already like "skinparam monochrome true" https://plantuml.com/skinparam#:~:text=Black%20and%20White. (You can go further with CSS and themes and...)
Another comment mentioned along the lines of, "it's the goto used by developers in readmes", and I suspect it's more specifically javascript-adjacent developers (as is the case where I work)
The "render locally" situation was enough friction to keep me happy with my .jpgs and .pngs generated from various sources and/or screenshotting.
I don't know if this helps you but the Mermaid plugin in JetBrains has an export feature which can save you a step. But I find Mermaid diagrams so limiting and the syntax more immature than PlantUML so it's very rare that I bother
The "in readmes" is a special case because the markdown rendering in both GitHub and GitLab support it without drama
