Managing negative keyword lists across 40 accounts - what actually scales?

Master plus local is the right model and shared lists at manager level do work for it. Two things that bite: 1) A shared list is applied per campaign, not per account. Adding a new campaign does not inherit anything, so somebody has to remember. That is the failure mode you will hit within a month. 2) The list has a size limit, and a master list serving forty locations grows faster than you expect. Prune it or you will find out where the ceiling is at the worst moment. On the account with a negative blocking a term you want: audit that before you centralise anything. Rolling a bad negative out to forty accounts is a much worse day than the one you are having now.

We do this for 22 accounts. Master list, one local list, and a spreadsheet with a date and a name for every term added to master. Spreadsheet sounds primitive but it is the only part that survived two years. Without it nobody can answer "why is this term blocked" and eventually someone removes it and the junk comes back.

The per-campaign point is exactly the trap I would have walked into. Auditing the local lists first before anything gets centralised.