You can trade perpetual futures, onchain, mostly decentralised, in self-custodial manner [1] e.g. on GMX

https://gmx.io/

Some more modern decentralised exchanges (DEXes) dealing with leveraged trades and try to minimise centralisation also include YieldBases:

https://yieldbasis.com/markets

There are other exchanges that are much more centralised, like Hyperliquid, and it is incorrect to call these decentralised. But there are truly decentralised alternatives as well.

GMX is not as popular, let's say Binance, because onchain user experience has been very hard. You don't want to sign every order from your crypto wallet. Transaction cost ("gas fee") used to be too high for trading. This is finally changing with the latest Ethereum improvement proposals, dealing with so called account abstraction.

[1] Because futures always settle on an external price, the price feed must come from some oracle. In the case of GMX, there are keepers (multiple of them) who are responsible to bring the correct price to Arbitrum chain and trigger the settlement. But it's not a single party.