Softfork

Softfork:-

In terms of blockchain technology a softfork is a change to the bitcoin protocol wherein only previously valid blocks/transactions are made invalid. Since old nodes will recognize the new blocks as valid, a softfork is backward-compatible.

Soft forks don't require any nodes to upgrade to maintain consensus since all blocks with the new softforked in rules also follow the old rules, therefore old clients accept them. Soft forks cannot be reversed without a hard fork since a soft fork definition only allows the set of valid blocks to be a proper subset of what was valid pre-fork.New transactions type are only added to softfork.

Softforks have been used on the Bitcoin and Etherum Block-chains to implement and accept functions.    

Source:- Investopedia