EIP-141 https://eips.ethereum.org/EIPS/eip-141 was introduced in 2017, but I cannot find it in any of the meta lists for forks https://eips.ethereum.org/meta. Is there a fork under which it was introduced or is that not applicable given any invalid opcode already doubles as an ABORT? Is there a list for soft changes such as this one?
Yes, any invalid opcode already doubles as an abort.
The point of EIP-141 is to guarantee to developers that no future hard fork will implement some non-aborting opcode at 0xFE.
It didn't require nodes to upgrade. It is not a change that requires a hard fork or a soft fork, because it is not a change at all. It is just the absence of a potential future change.