Please remember that we have provably fair games, these are considered to be the fairest games online. You can even verify the seeds of the games you played.
There's no need to be so negative towards casino operators that adjusts demo RTP.
Does "provably fair" apply to slot games too? I'm not entirely sure, and I don't have much experience with slots. I also don't know how many game providers actually offer provably fair games. I'm not talking about their internal controls and licenses, but a genuinely cryptographic provably fair mechanism, similar to what we have on dice or crash games, for example.
It does but not many, for provably fair slot list you can find some information here
Slots 102: The Provably Fair Slots (with poll)
At least there is one 3rd party provider with provably fair system if we look at the above thread, namely BGAMING and it is available in many casinos.
Most other provably fair slot games are mostly original games created by the casino such as Stake, Bitsler, Crypto.games, etc.
However I do not think that these provably fair system is popular enough compared to those probably fair.