I don't know if "Dallas", Unibet's exclusive NetEnt slot, is excluded from bonus play.
At "Dallas" you collect bonus money and when you have 5 bonus dollars the bonus game starts. The bonus game use your average bet during collecting spins. The player (if it's not excluded) can use bonus funds to collect 4 bonus dollars with high stakes and return with own funds and bet lower and still have a reasonable average bet.
It will be now you have mentioned it
It seems very much like TRII, where you have to collect 5 passports to travel the world for a payout of up to 450x average bet.
The downside is that it could be hundreds of spins before even the first passport is collected, and the last is hardest of all to collect (has to be unique, they are collected once for each reel).
It initially doesn't seem to offer a serious advantage due to the unpredictability of when all 5 will be collected, but maybe the players have now worked out a viable strategy that makes it a step too far for operators to ignore.
Unfortunately, more complicated slot games being developed means more opportunities to gain an advantage through the game mechanism. ANY game that carries value over from earlier bets is potentially a problem.
Unfortunately the seemingly random banning of otherwise ordinary slots has made a far wider audience aware that these opportunities exist, and probably have existed for some time, mostly undiscovered. This far wider audience now knows what to look for in future.
Developers will have to design such advantages out of the games, or implement an across the board blocking option for operators who use bonuses (which is probably all of them). Having the games blocked helps players because they can't break any terms that have suffered an "emergency update" because the operator "just noticed" the scale of the problem, and operators don't need to be forever paranoid about a similar type of issue going undetected, forcing them to drive WR to insane levels based on the drop in overall hold from their players.
However, there are also those games that have been banned where there is no such obvious problem, so despite this aspect having reached a wider audience, there is something else going on that currently has a much more restricted audience.