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Yeah, but for the vast majority of people sportives aren't really competitive in that sense, however they're competitive in that you'll probably get some dickhead trying to nudge you into the barriers in the finish straight in order to gain a couple of places/seconds.
AUK's stance is more concerned about en route support and that "hair shirt" Audax ethos.
AUK's insurers stance is more concerned about insurance. Having separate insurance to cover you whilst you ride the other event is doable, but presents a logistical/legal problem for AUK to rationalise.
AUK has also got its weird contradictory thing about 24h TTs where it actively wants people to ride the Mersey Roads 24h TT (because historical and to help support that event with entry fees) but then frowns upon awarding points for any other 24h TTs.
I don't think there's been a rule change, it's just in the past some people have frowned on DIYing sportives because they're grumpy bastards rather than there being any specific rule against them*. In practice you can submit anything and you're unlikely to be rejected.
(* There is still a rule on the books that bans drafting of people who aren't also entered into the same audax, which would disqualify many DIYs, were anyone to actually care, which they don't)