The modifier "$pushAll" was completely removed in the 3.6 upgrade by commit 5368ddee0e72af3283e2591f751a5b79925e64d0. Even if you have featureCompatibilityVersion set to 3.4, pushAll still is broken since the code is not there.
I believe this breaks the 'contract' of upgrading/compatibility set here: https://docs.mongodb.com/manual/release-notes/3.6-compatibility/. It seems to me that to have a truly 3.4 compatible version you would need to have this modifier feature flagged and then removed in 3.8 and the compatibility version is set to 3.6.
If nothing else, I believe that the documentation should be updated to reflect the fact that this does not work even with version set to 3.4.