Very few people internally to Apple cared about DTrace, and the person who was primarily responsible for evangelizing it left that particular role years ago. DTrace support has been slowly degrading over time (to the point that Apple has ended up shipping DTrace scripts on the system that do nothing but spew errors if you try and use them, as the probes changed and nobody updated the scripts).
It's a shame, because DTrace is cool and very powerful, but it seems almost nobody bothered to actually learn how to use it or cared about supporting it.
I've been using DTrace on High Sierra a bit for the past few months, and it has been working fine for me. I'm writing my own scripts though. I did notice that the sample scripts often did not work, but did not think that meant much.
DTrace is definitely cool and powerful. I'm surprised to hear that few people at Apple cared about it - I'd have thought it was an essential tool for them to solve certain kinds of problems. It's funny, because the tool itself is relatively simple. It seems to me that most of the knowledge needed to use it effectively is an understanding of the code being instrumented, rather than of DTrace itself. Hopefully the sample scripts degrading does not foreshadow the probes themselves degrading too.
I'm not just talking about "sample scripts". The system ships with a few DTrace scripts in /usr/bin as tools and they're rather broken. For example, running /usr/bin/iosnoop just prints an "invalid probe specifier" error. I haven't done an exhaustive catalog of all the /usr/bin DTrace scripts but last I checked there's at least one other broken one as well.
Trying to use DTrace with SIP also warns that "some features will not be available", though I don't know offhand what DTrace features don't work with SIP.
I think your problem with /usr/bin/iosnoop might be due to SIP. I no longer remember exactly which I did, but I know that I either partially or completely disabled SIP when I started to use DTrace, a number of months ago. I might have used one of the procedures outlined in this post [1] to do so. I do know that /usr/bin/iosnoop seems to work for me - at least, I just this moment invoked it, and got regular DTrace-style output with no errors like you mention. I am running an up-to-date High Sierra (10.13.6).
Edit: SIP is only partially disabled on my system, just enough to allow DTrace to work.
It's a shame, because DTrace is cool and very powerful, but it seems almost nobody bothered to actually learn how to use it or cared about supporting it.