just because you uninstall CFW doesn't mean there isn't traces in the logs.. The best way is to make a backup before CFW and restore that when you wanna go back to Vanilla. This way there isn't much they can see. Also I don't know if they can check your SD card, I don't think so but yeah..
Also:
<blockquote class="twitter-tweet"><p lang="en" dir="ltr">Nintendo bans for three things as far as I can tell -- all bans I've verified have been one of:<br>-Piracy.<br>-Cheating online.<br>-Dumping your SSL certificate and sending requests with it.<br><br>I don't pirate, don't cheat online, and this console's cert is undumped. I'm not worried
</p>— Michael (
@SciresM) <a href="
https://twitter.com/SciresM/status/1289702370922782720?ref_src=twsrc^tfw">August 1, 2020</a></blockquote> <script async src="
https://platform.twitter.com/widgets.js" charset="utf-8"></script>