Among salespersons, it means "that which we didn't sell you."

My definition of "legacy system" is something like "you can't ask for help when you need it on specific parts of the system".

This is true. I have heard security people argue that SMTP, POP and IMAP are 'legacy protocols' that must be ditched to better secure email.

The reason for the negative connotations is when it's used it also usually means "A system which nobody really understands", usually because of lack of tests and documentation. Maybe the high cost to modify it is working as intended in some cases (don't modify this project, start a new one). But when the business expects the legacy product to be modified, then you run into trouble.

To me it means existing code. If it has been accepted by the PO it is now legacy. The new code that was just accepted now influences any decision involving it going forward. Legacy.