Why can transactions use legacy gas fee model instead of EIP 1559?

Question

above is the adoption rate of the new EIP 1559 gas fee model.

So does it mean that you can still use legacy transaction fee model. If so, why is EIP 1559 a hard fork if it's not forceful.

Answer 1

Good question!

A hard fork doesn't necessarily mean the functionality wouldn't be backwards compatible. It means that users of the blockchain need to upgrade their clients, and that clients with the old version are no longer compatible with the blockchain. I would word it like this: client compatibility and function compatibility are different things.

Yes, you can still use the legacy transaction model, but it will not benefit from the newest changes. Here's some more info: https://eips.ethereum.org/EIPS/eip-1559#backwards-compatibility . You just can't use the legacy clients (they won't get synchronized).

Granted, it's a bit tricky to try to figure out what does it exactly mean that the EIP is backwards compatible. All the places just say "yes it's compatible, it just won't benefit from the new system". So I'm not 100% clear on that.