AFAICT, re-enabling these old OP-codes would require a hardfork.

If we had SegWit enabled, we could via a soft fork allocate new OP-codes for the same functionality (by introducing a new version of Script).
I believe the Elements alpha project has been experimenting with re-enabling old OP-codes: https://elementsproject.org/elements/opcodes/

2017-05-19 8:07 GMT+02:00 Mark Boldyrev via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>:
Back in 2010, there was a bug found in Core which allowed denial-of-service attacks due to the software crashing on some machines while executing a script - see CVE-2010-537.
I believe the removed ("disabled") opcodes should be re-introduced along with a standardized behavior definition.
For example, when execution of an opcode results in an arithmetic error, such as OP_DIV with a zero divisor, the script should exit and fail.
The string splice opcodes should also check their arguments for correctness, etc.

These opcodes would enhance the flexibility of scripts and allow sophisticated native smart contracts to be created.

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