Ublock Origin just introduced a way to detect third-party trackers disguised as first-party scripts: https://github.com/uBlockOrigin/uBlock-issues/issues/780#issuecomment-555643036

According to the issue it seems that it's becoming quite common for websites to use this technique to avoid adblockers detection, as you can see there is a big list of popular websites available here that are currently using this new trick: https://github.com/uBlockOrigin/uBlock-issues/issues/780#issuecomment-552971940

Could Pi-hole implement a similar feature as Ublock Origin but for domains?

EDIT: It seems that this functionality got quite popular because Adguard is considering adding a similar feature into their app: https://github.com/AdguardTeam/AdGuardHome/issues/1185 and the issue from the Ublock origin github repo is currently a top post on /r/privacy: https://old.reddit.com/r/privacy/comments/dyrhg1/to_prevent_thirdparty_trackers_disguised_as/.