Right, I'm getting through so much tin foil this week there's shortly going to be a worldwide commodity price hike....
Firstly, we know that platform software supplied to the casinos by third parties is fixed in TRTP and gameplay, excepting RTG's '3 choices' software.
Next; Imagine a land mechanical slot - fixed physical reels, and separate stops that produce a 95% TRTP. For that to be adjusted, the reels would have to be changed via more/less symbols of one kind or another, a complex operation as the paytables would presumably remain fixed.
OK, this exact same slot above but online: For speed and effect separate reel-stops aren't necessary - the RNG picks an outcome from the pool and a subsequent graphic reflects this. The reel strips and paytables are exactly the same, as is the 95% RTP. I know some don't agree with me here, but most players accept that this is why the reel graphics seem 'gimped' on most software, especially WMS and 3Dice. It's because a win value is selected, then a graphic is produced to represent ANY reel combination that can produce that particular outcome.
Now, we want to change this ONLINE version from 95% to 90% RTP. The beauty of online slots is that NO adjustment to the reel-strips would therefore be necessary - simply the 'pool' of outcomes is tweaked to produce more low-return or zero results. The RNG works exactly the same, but in the long term will only produce a 90% RTP.
In computer program terms this would be easy, simply making a small edit in one line of master code.
So, to answer Weesies point, is it possible? Yes, absolutely. And possible to enact for the whole game or just one person's pool of values? Yes, easily.
Now the moot point is that 99% of sites rely on fixed 3rd. party software which is independently verified, so it would not be an option for the casino to adjust it, but only to choose a tighter/looser pool for their games in the case of RTG. The other softwares don't offer the RTG choice.
However, in the case of a site offering it's own bespoke software i.e. 3dice then the chance is there. (I use this example because 3dice springs to mind as a rare site that uses and offers it's own bespoke games and there is no insinuation that this impeccable operation has or would do such a thing!)
IF a site did this if it could, then it would be in breach of every licence and software verification going and would be licensed in bandit-country.
So, possible? Very. Does it actually go on? I'd hate to see it proven.