What is fTestnetToBeDeprecatedFieldRPC seen in chainparams.cpp? fTestnetToBeDeprecatedFieldRPC is a bool type variable set to some values in the main, testnet and regtestin chainparams.cpp. What is the use of this value ?
The field corresponds to false for mainnet, true for testnet, and false for regtest. You can think of it as a flag that tells you whether the current network is testnet.
Why's it named like that?
To get people to not use it, and because getinfo and getmininginfo still use it:

jtimon commented:
Yeah, comparing to the testnet id was very readable so it's the new flag. The difference is that when people see &quot;NetworkID() == something&quot; they think they can use that too, while FlagWithLongNameIncludingTheWordDeprecated() is not as likely to be used or that's my hope.
@laanwj explained the deprecation thing, but basically, RPCs can just use bip70's networkidstring()
For the string, I was going to create another flag but @laanwj wants to keep chainparams clean of GUI-related things. Of course comparing to an enumerator is better than comparing to a string, again, the goal is that people don't see the comparation to the enumeration and think they can call CChainParams::NetworkID(), hopefully the comparison with a string will make people use that approach less, but we need the string for BIP70 anyway.

(Source.)