The metal oxide varistors in there degrade as they soak up overvoltage events. They typically have a thermal fuse in front to protect from bursting in a high energy event or overheating as their leakage current gets too high. If the light is off, my assumption is the fuse tripped and there is no power going to the the protection circuit. It would be likely safe to plug in (you can have it plugged in in an open area and check for any hot spots to be sure), but isn't going to protect any downstream components. If you've already got something upstream of it, maybe that doesn't matter - I'd also make the determination based on what you expect to plug into it - since you're planning on hooking up TVs and systems, I'd probably just buy a new one and save this for people to plug in cell phone chargers or something elsewhere.
By the way, it depends on the power quality you have, but the rough rule of thumb is to replace surge protectors every 5 years (or immediately if there's some evidence they clamped a big event). I'd again base this on what you're actually connecting to it. If you had really high dollar stuff, I'd also look at getting something for the service entrance - it's actually mandatory in the NEC as of 2020. That'll be way more effective than these power strips anyway.
