Stop leak, and leak detection dye.
One of these is a diagnostic tool. The other can make a system nobody will touch.
Two different products get confused because both go into the refrigerant circuit through the same port.
Leak detection dye is a fluorescent additive. It circulates, escapes wherever the refrigerant escapes, and shows up under a UV lamp as a bright spot at the leak. It is a diagnostic tool and it is widely used.
Stop leak, or leak sealer, is a chemical intended to react where refrigerant meets air or moisture and plug the hole from the inside.
Why stop leak is a problem
The mechanism that makes it work is the mechanism that makes it dangerous: it hardens on contact with moisture and air. A refrigerant circuit is full of places that is undesirable.
It can plug the metering device. A restricted TXV starves the coil and looks exactly like a low charge, which is how a sealed leak turns into a worse problem that gets misdiagnosed.
It can plug the filter drier and the capillary passages.
It contaminates recovery equipment. This is why it matters to you even if you do not care about the technical argument. Recovery machines and gauge sets cost real money, and sealer can ruin them. Plenty of companies will not work on a system that has had sealer in it, and some ask before they connect.
So a product sold as a cheap fix can leave you with a system that is harder to get serviced, by anybody.
Where it is defensible
Not never. A system scheduled for replacement where somebody needs it to limp through a fortnight, with the owner told exactly what is going in and what it means, is a judgement call rather than a scandal.
What is not defensible is putting it in without saying so. If a system has had sealer, that fact should be on the invoice, because the next technician needs to know before they connect equipment to it.
Dye is a different conversation
Dye is a legitimate tool, with two caveats worth knowing.
It takes time. The dye has to circulate and then escape, which on a very slow leak can mean a return visit days or weeks later. A shop that adds dye and asks you to call back is not stalling.
And it is additive in an oil-sensitive system, so it belongs in manufacturer-approved quantities rather than poured in by eye.
What should happen instead
A leak is a location. Finding it is electronic leak detection, a bubble solution on fittings, nitrogen pressure testing, or dye, and Schrader cores and service valve caps are checked first because they are the cheapest common culprit.
Then the repair, then evacuation, then a measured charge with superheat and subcooling recorded.
If somebody added refrigerant to your system without searching for a leak, that is a purchase rather than a repair, and the scam guide covers what to ask next.