Halon was phased out for commercial use around 2000 if my memory is correct. It is a very effective extinguishing gas, but it is also an ozone depleter. The military has stockpiles of it, and still use it for their legacy equipment.
Technically, it should be serviced annually, but not many people do it.
if your agent is still full and the system appears to be operational there is a good chance it will work. I am not condoning not testing it, just giving my opinion.
if you decide to replace it, Fireboy makes a good replacement agent. You can buy the cylinder, based on the engine room volume and hang it in there. It has a fusible link that will melt, and deploy the agent, automatically if there is a fire.
For smaller size boats like ours, you are not required by the USCG to have an auto system in your ER, just the required amount top side. Personally, I want one down there. I therefore pulled the oem dry chemical, and put in a clean agent Fireboy, in addition to the clean agent extinguishers I have in the galley and cockpit. A fiberglass boat will burn down to the waterline fairly rapidly, so I like to have all of the fire power necessary to give me a fighting chance.