Ephemeral in thoery...except that most LN node software saves and stores transaction info by default, and you also don't control what info other nodes save. Peter Todd did a talk on this problem a few months ago showing how his node still had transactions he made over a year ago.
Onion layers aren't a magic barrier. Tor has a long list of attacks that can reduce or completely deanonymize you. Now consider that even if you use LN in a private way (vast majority do not) less than 1% of lightning nodes hold over 99% of total liquidity...
https://www.youtube.com/watch?v=r3iFrkESigo
https://github.com/Attacks-on-Tor/Attacks-on-Tor