I'm not the expert on this, nor am I an expert on the timeline of the rule changes since DPF (and DEF) were mandated in the US. All I think is that through the EPA, the Obama/Biden administration tried to crack down on deletes by targeting the mechanics who were removing emissions systems from diesels - not the drivers having them removed. When Trump was elected, at some point, despite rules remaining in place, the EPA laid off enforcement for the most part. I'm not sure if there was much of a change during Biden's presidency. Today, as far as I'm concerned, the rule is still technically in place. If that has changed, recently, I'm unaware of that. HOWEVER, there's very much a don't ask-don't tell policy if you have a deleted system. And mechanics who enable deletes are still running the risk of being caught, though the risk is much much lower than it used to be. Where you are probably has a lot to do with it as well. You're much safer in Texas than California or Colorado, for instance.
The number of people who delete is higher than most think, but many are a bit hush hush about it. I've talked to three diesel specialists who've said all the emissions equipment hurts performance and is detrimental to longevity - it's important to understand that notion is a generalization. Under good conditions, one can probably make their brand new shiny diesel go pretty far, but still likely not as far as some of those 7.3L Powerstrokes from back in the day. Most will sell before then anyway, so perhaps it doesn't matter from a practical standpoint. If you're like me, philosophically, it's a bummer to see companies engineering these incredibly powerful engines, then bolting on extra equipment that downgrades the product (from an engineering perspective). Remember, it does make the tailpipe emissions cleaner
