Where can I find a record of blockchain soft forks?
Bitcoin Core maintains a document with all BIPs that are implemented, including those which establish a soft-forking consensus change. From that list, BIP16, BIP30, BIP34, BIP65, and BIP66 have been soft forking changes.

In the two first years of Bitcoin's history, changes were made more frequently, and much less care was taken that all consensus changes were backward compatible. Most of them probably were, though.