Bitcoin-core already applies the low S-values as a standard rule for the transaction relay, but making a nonstandard transaction causes a softfork.
BIP-146 claims a softfork by invalidating high S values in tx signatures, not just non-standard ones.
The PIF also indicates that the majority of transmitters on the network already adhere to the bottom S rule, as of August 2016. To quote an excerpt:
The reference client has produced LOW_S compatible signatures since v0.9.0 and the LOW_S rule has been applied as a relay rule by the reference client since v0.11.1. As of August 2016, very few transactions violating this requirement have been added to the chain. For all currently usedPubKey script types, non-compliant signatures can easily be converted into compatible signatures, so these requirements do not result in any loss of functionality.