We need support PlantUML
Since PlantUML requires Java to run it will probably never been built into RemNote.
I think you have to use an external application to generate the image which you can then paste into RemNote.
Mermaid.js also supports some diagram types. Have a look here: Flowcharts and concept maps
The primary difference is with PlantUML, we can only storage text on remnote server and users can edit text in Remnote directly.
If RemNote server not easy to intergrate Java, It also posibble to run an independent PlantUML Java server by RemNote , just to convert text to image at runtime when user visite notes on web, There is no association to current RemNote server.
There is another way to support this:
Let users can setting a PlantUML server url by themself.
But I think may be PlantUML is not a common thing for majority users, Maybe use a plugin to implement it is nice.