How do the most popular Bitcoin wallets handle transactions stuck in the mempool with too low fees? If unconfirmed for a specific period of time, do popular wallets remove transactions from the mempool? Are there some wallets where if your transaction is never confirmed, your funds are never returned?
In general TX is removed after arbitrary time period (14 days for Bitcoin Core) or when mempool size is exceeded and space is needed for fresh TXs. For Bitcoin Core check this answer.