I have seen way more this last year too but it may be that I am looking closer and may have missed some before. I check every O2 sensor where it threads in and see plenty of spacers on Subarus and Jettas mostly.
People are usually shocked when they find out I'm not a very good electrician.
This year I've failed a lot of cars for having 02 spacers. This is in San Gabriel valley. it can be difficult to see at times, a mirror on a stick can be useful to spot it.
yea the amount of custom spacers available has increased, before it was only the diy person that would make one out of plug foulers, now most are sold on amazon or other websites for very little money.
I hate smogging so called craiglist,sell a car in a corner, car dealer, type of customer. I had one of those customers and overheard him tellimg another customer he would ad the o2 sensor extention, then run the monitor for cat, then remove the modification and as long as he dont go on freeway the smog passes.
I dont have manny of this customers but I have some. So if you smog as presented the car passes, but I always said if you have alot of car dealer customers your FPR will take a hit and you cant do nothing about it.
Theese guys think up shit like this and the regular customer does not, only car flipers,and shade tree mechanics. they were leaving the extenders on and cars were failing as tamper, now they take them of and drive slowly to get smogged, so code will not set. they will do anything to pass instead of dropping 300+ on a new cat