Tim Beiko explained that Ethereum’s community testnets, like Mekong, would provide the final opportunity for practical feedback before upgrades reach the mainnet

Quotes that support claims

Following headliner selection and initial implementation, a dedicated ephemeral testnet (similar to Mekong) is launched to provide an implementation testing ground for the community. This should be the “Last Call” for the ecosystem to engage with a feature and propose spec changes. After this stage, the expectation is that only substantial security issues or severe unforeseen problems will delay or halt deployment.