Ah… since this thread has come back to life, here is an update.
Canon told me that they recommend a filter to completely weather seal a lens. The black ring around the end element of the 24-70 is held on with industrial strength double sided tape, basically. If the seal isn’t perfect, junk can be sucked in under that ring when you zoom the lens. That also explained why 90% of the accumulated crud was around the end element.
When covering fires and working in other extreme environments, I use filters, the rest of the time, I don’t. That has helped cut down the debris sucked into the lens. Over a few years of using them, all of the RF zooms have accumulated some dust mostly on the end elements. It seems like more dust than I ever had with EF zooms, but to be honest it could just be that I’m looking more, or just more aware of it now. It’s not enough dust or large enough pieces that it has any impact whatsoever on image quality (unlike the original problems I has with the 24-70) or lens function. The one zoomer that is still dust-free is the 15-35, which is probably good since it would be the most likely to have stuff showing up in images with a buildup of dust.
It’s also funny that this thread came back to life just as Canon announced the 24-105 2.8 that has internal zoom and should suck in less dus that way. I have already put an order in for one.
Anyone looking for a nice RF 24-70 that has been serviced regularly by Canon? ;-)
