PSync

My current favorite is SipHash.
This benchmark says xxHash is faster than SipHash on short inputs, although I don't know the collision situation.

Currently we use Murmurhash3, but newer and faster hashes are available

More importantly, I'm not sure if Murmur3 guarantees that the computed hash is the same on all architectures (e.g. big/little endian), so using it in a network protocol is a major issue. Other modern hash functions such as xxHash guarantee portability.

CityHash (NFD uses it), FarmHash, xxHash

If you're going to change hash function, I don't think there's a good reason to use CityHash or FarmHash today. xxHash is probably fine, and also take a look at SipHash and HighwayHash.

https://gerrit.named-data.net/c/PSync/+/6599

So the current patch-set removes Murmurhash3. Is there any utility in keeping the code there for reproducing old results and for comparison purposes?

If you need to reproduce old results, check out old version of code.
Fewer/single choice => fewer moving pieces, less code to maintain, less confusion to developers.