Draw.IO is now called diagrams.net. It is an excellent cross-platform diagramming tool with an online version and downloadable application.
Backwards compatibility for diagrams is a key feature of Draw.IO. “Diagram files created in 2005 will load in the app today” is a key message on their web site.
Draw.IO rivals commercial options such as Visio. Draw.IO comes with a full set of UML 2 and UML 2.5 symbols.
There is also a Visual Studio Code plugin that allows Draw.IO diagrams to be edited directly in your IDE. See Henning Dieterichs’ GitHub repository here for details including installation instructions.
This Draw.IO plugin for VS Code can edit
*.drawio.svg files. The later two generate images viewable directly in a browser.
Many of the illustrations in this collection use drawio.svg files to generate the image, along with PlantUML.