I, and I'm sure everyone who was there, remember David Burn's lecture on takeout doubles at the Summer Congress in Brighton, probably ten years ago or more now.
He was addressing the 'British disease' of needing 5-card support for the unbid suits before you made a takeout double, something that he attributed to the writings of certain experts of the older generation.
He contrasted it with the Italian approach, where the double tended to promise little more than opening values, and the European approach where double tended to promise values with support for all unbid majors.
In particular, he told of how the Swedes are taught that when they hold opening values with four spades, and the hand on your right opens one heart, then you put the double card on the table. It was thought that their babies did this before learning to walk.
With the values for opening bids dropping drastically, getting into the auction quickly is becoming more important and people are doubling now with hands that used to be thought unsuitable. I would say this is 'expert' standard in Europe now and is independent on whether they play equal-level conversion or not.
However it has done little to resolve the debate over whether to double or overcall when holding 5-4 majors
Auction
1♥ X 2♦ 2♥
all pass