Joplin
Close matchFree with paid extrasFree and open source, and worth a look if you want more structure than Simplenote offers, such as notebooks and attachments.
Your files: Imports plain text and markdown files exported from Simplenote.
Does Simplenote run on Linux?
Verdict: Yes
Yes. Simplenote lists Linux among its supported systems and publishes a Linux download, and there is a web version too. Your notes sync from your account, so nothing needs moving.
Simplenote's own site lists iOS, Android, Mac, Windows, Linux and the web. The Linux download is published through the project's code repository, and there is a community package on Flathub as well, though that one is not verified by Automattic.
Because everything syncs through your Simplenote account, switching machines is a matter of signing in. Every note, tag and version history entry appears. There is no export, no import and no file to carry across on a USB stick.
The web version is a genuine fallback rather than a stripped-down one. If you would rather not install anything, app.simplenote.com in any browser does the same job, and Simplenote is simple enough that the difference barely registers.
The app is still free, including the syncing and sharing, which is worth saying because so much note software has moved to subscriptions. What you had on Windows is what you get here.
Simplenote on Linux, and alternatives
Obsidian
Different way of workingFree with paid extrasKeeps notes as plain files in a folder you control, with links between them. More to learn, but nothing is locked in a service.
Your files: Reads markdown files exported from Simplenote straight into a vault folder.
Last checked . Sources: