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Yes, and yes - the LPC bus is standardized, and TPMs need to use the same protocol to enable them to be modules on the motherboard.

And yes, it's only $10 or thereabouts for a logic analyzer that can decode LPC bus communications.

2019 ref - https://pulsesecurity.co.nz/articles/TPM-sniffing

Some processors have a 'Firmware TPM' or 'Platform Trust Technology' where the TPM is integrated into the CPU itself, meaning no wires to probe/interpose.

RAM, PCIe and USB remain unencrypted though so you can just interpose one of those if you're performing an evil maid attack.

Recent Intel/AMD CPUs feature transparent full memory encryption.

Recent AMDs have another feature called SecureBio(metrics) which directly connect one USB socket to the CPU in such a way interception is not possible, which allows one to design a secure USB device (fingerprint reader, webcam for now) which can't be intercepted at the bus level.

PCIe is still vulnerable, but I bet it won't be long before transparent encryption comes to it.

Many of them use LPC, but LPC isn't the only protocol used, as I found out in my attempts to find a TPM for my desktop.