Or insulation on good expensive test leads.
Fixed that for you (semi-kidding).
Most of the affordable and usable test leads I have, are just overly plasticized PVA. The difference is that that stuff does burn at temperatures where silicone doesn't.
Aside from lower maximum temperature, the downside is that the plasticizer tends to leech out if in contact with a compatible plastic, and completely meld the lead into that plastic. So they're definitely usable and perhaps even
good, just not as good as proper silicone test leads.
I wonder how many have tested their test leads whether they really are silicone or just overly plasticized PVA? I bet many have the latter, while believing them to be the former.
Apologies for the repeat-peeve (I already mentioned this way back in this thread).