If you don't mind me asking, what was the seminar about and who was presenting it?

The information I have seen supports your statements.

The seminar was aimed at architects and the topic was 'refurbishment of existing housing' with an emphasis on solid as opposed to cavity wall construction. You are probably aware of the trend towards applying PIR/ PUR panels to the internal faces of walls in order to improve thermal insulation performance. It was claimed at the presentation that this material shrinks over time. The manufacturers deny this. I really don't want to give out the name of the presenter - I am confident that he said what he said in good faith and just wanted to follow up and try to verify it. The supposed shrinkage and supposed off gassing are obviously parts of the same process.

The big questions on shrinkage would be "how much" and "how soon". I have seen references to EPS shrinking as well (it was most recently mentioned in a post in the ICF forum about block shrinking). I would think sheet stock would finish any outgassing/shrinking within a reasonably short amount of time (large surface area to volume). Most issues I have seen reported seem to be from foam-in-place products instead of premanufactured sheet/stock.

Bigrig;

I have never heard or seen a reference for EPS shrinking, it is one of the most stable building products out there and it does not loose R-value with age like PU.
Sheet and block PU do continue to offgas for a long period of time. one of our manufacturers produces both EPS and PU laminated panels, they have had a moratorium on the PU panel production for the last 8 months until the offgassing issue is resolved.

Mayhaps he was thinking of XPS shrinkage? (Which is a known phenomenon, but much-improved since the 1970s). All the shrinkage that will ever happen with EPS occurs in the minutes that it cools from molding.

The R-loss of polyurethane is required by FTC rules to be already accounted for in the advertised R value. They might specify R6 or R6.5 per inch, but it'll likely perform to R7-R8 right off the presses, only sinkng to the "aged value" over a decade or more.