Also, transmitting MAC addresses and IP addresses in the clear really isn't anything to write home about -- that's how all TCP/IP packets are transmitted over ethernet, after all. The real question is what they do with that data on the server side. If they so desire, they could change that behavior far more easily, and retroactively apply that transform to all the data they've retained.
Not even 'usually', it doesn't exit your LAN at all, once it hits your router, it is stripped and your data is packed with a different MAC address on the WAN (or any other layer 2 identifier depending on your connection), same for each hop after that.
If you use EUI-64 as your interface ID in IPv6, your MAC address is part of your IPv6 address. That said, no modern OS does that anymore, times 1% of IPv6-enabled users, so the closer term is "almost never".