Yes, at least so it looks in src/main/objects.c
Also, we had a very similar change a while ago :
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r65926 | luke | 2014-06-12 15:54:38 +0200 (Thu, 12. Jun 2014) | 2 Zeilen
Ge?nderte Pfade:
M src/library/base/R/datetime.R
Commented out class(x) <- NULL in [.POSIXct and [[.POSICct.
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and we never seemed to have followed up in a systematic manner
finding other places where this happens and could be
eliminated. I see about half a dozen examples in
base/R/dates.R alone and am trying to find more in other places.
[maybe this used to be necessary for very early different
versions of NextMethod() which were not yet optimized using .Class etc]