Revert "DEV: Drop old IE11 intersection-observer references" (PR #13017)

This reverts commit 7360a0f70fae59bb065dffb9b1ad42d13ff38f46.

iOS still wants this sometimes. Probably best to revert for now and we can always remove this again later.

See: https://meta.discourse.org/t/189799/11?u=blake

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