Without SSL on the LAN side of a reverse proxy, I presume that all traffic between the server and the reverse proxy is unencrypted and, thus, accessible to any device on the LAN.
Which specific scenarios result in this being a concern? The primary concern that I can come up with is if you know that there are untrustworthy entities connected to the LAN (untrustworthy devices, or perhaps malicious individuals).
Only if that traffic is using broadcasts. Wired networking on moden hardware is strictly point-to-point, PC1 is completely unaware of any traffic between PC2 and your home server or whatever.
Wireless is different and can ostensibly be snooped by anything that knows your network key, but I’d assume that you’re not running services on wireless devices.
TIL of PPPoE! Could this still be circumvented through ARP spoofing, though?
I wasn’t speaking about PPPoE specifically when I made my post, all wired ethernet traffic only travels from sender to recipient without being visible to any other devices that’s not in the direct communication chain. This wasn’t always true. A network hub will send out incoming data to every single port, but hubs haven’t been in common use for decades. A network switch is aware of what is plugged in where, and will only send received data out whichever specific port the destination is connected to. If you have three PCs plugged into a network switch and PC1 needs to send a packet to PC2, PC3 has no way of even knowing it happened.
That said, your final point is correct, and ARP spoofing defeats this. It had completely slipped my mind when I made the above post.