To my knowledge typically only a quite a small number of people still use the previous major version compiler at this point in the major release circle. (say roughly at the x.y.4 point, as 3.2.2 is well established now)
The significance of "quite small" can be argued in any context. Only a "quite small" number of people use Pascal, but I'm sure that they would be very unhappy indeed if e.g. Linux started to mandate code signing and the developers only supported majority languages.
The specific case I was thinking of though would be if somebody needed to support multiple distro versions which bundled multiple versions of libqtpas for Qt support, which would have implications on the versions of the LCL hence IDE hence compiler to be supported.
But in any event, it is- IMO- completely unacceptable for a program to fail to compile on even an ancient compiler version, without a sensible error message explaining /why/. In fact I'd go so far as to say that that would be a far graver sin than anybody had admitted to so far.
MarkMLl