I've just heard about a staking model which doesn't raise the supply. what??
can somebody tell me if this is technically possible?
Yes ofc.
It is even possible to burn coins.
Casper suggests that. Stakers could be rewarded just by fees, and some of those fees even be burned, reducing the total supply over time. This is suggested by vitalik.
Because of the lack of high electricity consumption, there is not as much need to issue as many new coins in order to motivate participants to keep participating in the network. It may theoretically even be possible to have negative net issuance, where a portion of transaction fees is "burned" and so the supply goes down over time.
https://github.com/ethereum/wiki/wiki/Proof-of-Stake-FAQ