Worsened by the fact that you get an insurance discount if you have a car alarm. So everyone has one, despite that we all know they are only effective at annoying neighbors and bystanders. The only writing I've read that touts the positives of a car alarm fall in the "wishful thinking" category, a.k.a, "well, it can't hurt!" Everything else I've read that tried to be at least a little scientific indicates they're more trouble than they're worth.
On the other hand, it's possible that the reason car alarms are mostly false alarms these days is due to the huge decline in auto theft rates over the last 30 years [1], which is due at least in part to the prevalence of car alarms.
30 years ago I could open a locked car, and get it started and driveable in sixty seconds (the steering wheel lock will be collateral damage). Not a car thief, but I was a mechanic. But now there are better door locks, a lot of cars don't even have a key-locked wheel lock, and you don't just short two wires under the dash to start it like the movies. Granted, a career thief probably has quick workarounds for all of this, but the bar for quick car theft went up from "low-life with a slim jim", I think.
Point is, yeah, car alarms likely had some effect, but was it more or less than the fact that cars are just generally harder to steal now? And I have no idea what the answer is.
It's also possible it's because the number of movies Steven Seagal starred in for the past 30 years has also declined [1].
Car alarms can deter joyriders but don't do much for the professional car thief [2] ([3] has more references). Also, as an anecdote, there's been a bunch of car prowls in my apartment building parking lot for the past year or so. The victims were high-end vehicles with valuables in-sight (briefcases, dashcams, etc..) and they all had car alarms.
Lots of merchants sell non-street legal straight mufflers with a wink wink nod nod description. In the Internet mail order age it's even easier to buy one.
Saying "for off road use only" is just to reduce liability in states that happen to have byzantine restrictions on whatever you're selling. You don't want someone to sue you because you didn't tell them that the thing you sold them wasn't legal in their state or because a law changed that you weren't aware of.
Most straight through mufflers are street legal because most states just have a law about X decibels at Y distance and don't care what you have as long as it's quiet enough. CA has quite asinine rules about car exhaust systems though and you could be considered not in compliance with emissions rules even if you somehow make your straight through exhaust reasonably quiet.
To me, this is the biggest problem with car alarms. When people hear one, they don't think "A car is being stolen" they think "Ugh somebody accidentally triggered that".
