There’s a cheap ($20?) add on for our water heater that adds a conductivity sensor in the drip pan, and a shutoff valve on the inlet.

There’s a slightly more expensive variant from another company that supposedly looks at flow for the whole house, and cuts water off due to slow leaks or burst pipes.

I’m shocked that the water heater cutoff valve isn’t required by code. In contrast, plumbed drain lines for water tank flushing have a much worse payoff per dollar and are required by code around here. (When I’m feeling cynical, I assume that the codes here are designed to subsidize contractors while maximally screwing over home owners.)

Edit: the plumbed lines don’t actually help with leaks; so they do nothing that a garden hose would not accomplish.