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Now, I'm sure I am in a very small minority of advertisers that this actually would apply to as I exclude TONS of sites.
The good news is that if you currently exceed the new limit you're fine you just won't be able to add any new site exclusions until you bring the number of excluded sites under 5,000.
The reason cited for the change is the introduction of Site and Category exclusion tools.
The one question I had was whether or not Quality Score could be impacted if you have too many excluded sites. Ridiculous to even ask but you just never know.
No word back yet.