I doubt very many Council members were victims of burning at the stake, given the difficulty a mob would have catching someone who can ward their home, veil, and retreat to the Nevernever if cornered. The mobs probably mostly got random wrongfully accused vanillas or innocent small-timers, with a scattering of genuinely malignant sorcerers and warlocks.
Or more than a scattering.
When you consider the 'rules' of how the DV works, the more you consider it, the more likely it looks that a lot of the victims of the witch-burnings may have been guilty as charged of nasty actions.
As JB notes, a wizard-level talent might come along in a given country back in the day maybe once in 3 generations. Bigger countries would have a few more possibles, smaller ones a few fewer. But still pretty rare.
So you've got a small Council, slow communications, hard travel. Most magical talents are therefore going to be either self-educated or taught by whoever local is available. Further, for a person with modest powers, a lot of the most immediately useful things you can do with it, the stuff that would occur most naturally,
takes you into black magic.
You're a poor farmer, barely making subsistence, illiterate, with a large family to feed. You don't own your land, you just farm it, and your local lord, or later your local landowner, takes a bit chunk of the crop as payment for being allowed to use the land. In some periods, it may well be literally illegal for you to leave you manor or land area.
(If you think I'm kidding, I'm not.)
But it just so happens that you can
force your local landlord/noble to give you a break on your dues and fees and boon work, and even have him think it's his own idea. All you have to do is change his mind...
Or you're a small villager, an apprentice of somebody, and bandits are raiding the town, stealing the meager prosperity, harassing the women, or worse, and you've never used a weapon in your life. But it just so happens that you can kill someone by concentrating hard...
The thing is that there's going to be a nasty tendency for small practitioners to end up sliding into black magic, even starting from natural, perfectly understandable motivations. Even if they don't know why they shouldn't do it, the effects still follow, and pretty soon you've got a warlock loose.
Add in the fact that not every practitioner starts with good intentions, and suddenly it becomes likely that a very significant percentage of them end up going to the bad. Probably the majority of practitioners that people actually get much direct experience with were Trouble.
So it's a good bet that in the DV, a lot of the people executed for using magic to do various evil things...actually did those evil things. Not nearly all, of course, but a significant percentage were probably guilty.
The Medieval and immediate post-Medieval era tended to see magic users as in league with the Devil...and you know what? In the DV it's a good bet more than a few were! Even Harry, in his younger, stupider days, consorted with Chaunzaggoroth. One way for a minor practitioner to get more power, even with good intentions, is to find a 'reasonable, sympathetic' contact like Chaunzaggoroth to help them...bit by bit, compromise by compromise, they end up being what the mob thinks they are to start with. Harry was not nearly as desperate as some of the people who would have been calling up such beings back in the day, and he still came close to getting ensnared.
But yeah, a lot of their defense probably also did rely on concealing what they were (and, as Harry mentioned at one point, propagating myths that magicians needed elaborate props like crystal balls, so, hey, obviously we aren't magicians, right?) rather than, say, advertising in the Yellow Pages. And if, as several of us were discussing recently on another thread, some of the unsolved assassination attempts on him have been the Librarians probing how hard it would be to kill a wizard after Harry outed himself as a testable specimen, the orthodox Council view on discretion might still have its merits.
It might well. It's not by any means clear that Harry is in the right in his 'highly public' approach. It's not clear that he's totally in the wrong, either. It's too soon to tell. But the fears of the older Wizards are by noi means unreasonable.