Well, if any gambling authority fines Stake.com and they will not follow the procedures this will have consequences.
If this case will be officially acknowledged by the authorities as a violation of stake.com, then no doubt that they would have to face the consequences and compensate you for all the damages that they have inflicted. But, until that happens, we could only wait then.
Well if they acknowledge their mistake there might be a violation, but this kind of violation does not result in them getting penalized by the authorities. The way I see it, it's just a simple mistake, OP has also not figured out how to stop from receiving an email that is very visible in the first place.
They have already acknowledged their mistake and blamed their database and said it had errors. While you might see this as a small mistake, we all are not aware to how many problem gambler Stake is doing this. Problem gamblers are probably their main source of income and they should not target excluded account for their promo mails. There is no casino that is allowed to do this.