It's not in the source code, but instead in a subscription that ABP uses by default (it remembers if you unchecked "Allow some non-intrusive advertising" so it doesn't re-enable this list on every update): https://adblockplus.org/en/acceptable-ads

This is the subscription, which was originally linked from that page (but it now links to the Acceptable Ads subforum, which has a sticky that links to this subscription): https://easylist-downloads.adblockplus.org/exceptionrules.tx...