Unless I misunderstand your question, draw.io can be downloaded as a standalone Linux application and run locally.

Likewise, the Xfig package should he available in most Linux repos. It’s old, but good enough for a quick sketch.

edit: aha. My mistake. My eyes slid over ‘open source’ in the title*, and even still I hadn’t realized it was an Apache license.

* Whaaat, it was pre-coffee? Let the purest among us cast the first stone.