Do not show the tokens from Ethereum mainnet in the 'Add token' dropdown list when the user is on a custom network (like BSC, etc.). Continue to show the Ethereum mainnet tokens for the test nets (Ropston, Rinkeby, Kovan, Goerli).

Context on tokens being network-specific

DAI needs to be minted separately on different chains - they aren't equivalent, they have different token addresses. User is trusting the network when they are adding a custom token. So, Matic, xDAI, BSC all have 'DAI' (with different token addresses), so users who are trying to move DAI from 1 network to another, then you have to go via a bridge (an application that interacts with the 2 chains to make this transfer of assets).. BUT, users don't know they have to do this via a bridge (NOT MetaMask) - if they do this without bridge then users lose their money.