Yes, I know, draw.io theoretically isn’t entirely open source, but the source code is available and it can be self-hosted. Honestly, that’s good enough for me, I think I can make an exception for this one. But generally I care a lot about strictly using FOSS too. It can also be integrated with Nextcloud: https://apps.nextcloud.com/apps/drawio
Corporations steal from us all the time, and they don’t even let us buy their content, they only sell limited access that can be revoked at any time (see https://lemmy.dbzer0.com/post/9531016 or https://lemmy.dbzer0.com/post/12904663). Under these circumstances, there are no ethical issues with piracy.