OP_LSHIFT & OP_RSHIFT purpose & functionality

For all intents and purposes, Bitcoin has no such opcodes.

Very early versions of Bitcoin's reference implementation did, until version 0.3.10 (August 2010) when they were disabled together with a number of other opcodes. New opcodes with identical functionality could be re-introduced now with a softfork, but the fact that similar opcodes existed historically wouldn't make that any easier (or harder) - they'd just be new opcodes.

As for what they did: they implemented an operation similar to the >>= and <<= C/C++ style operators, multiplying and dividing by a specified power of two.