Is your feature request related to a problem? Please describe.
SET default_transaction_isolation... and SET transaction_isolation... both return SET indicating they've taken effect, though they cannot be meaningfully set to any value besides serializable.

Describe the solution you'd like
I think any kind of change to behavior would be an improvement:

• Remove default_transaction_isolation and transaction_isolation as settings available to the SET statement.
• Return an error indicating the setting is read-only.

Describe alternatives you've considered

Additional context
It's challenging to document the relationship between SET and SHOW for settings that are essentially read-only.