One thing to note/add since you incorporate decentralized apps in a permissionless blockchain. The concept of DApp itself is built based on smart contracts. The blockchain itself doesn't care about who is accessing it, it only follows the consensus rule that is based on the majority.
On the other hand, a smart contract is just executing predefined terms/contracts. Right here is the part where you can restrict the end user.
I am studying DeFi theoretically and was wondering whether these combinations are possible in praxis or whether this is excluded by definition.
Open access on permissioned blockchain is may technically possible, but it will dismiss the definition itself.