21 days is not a default setting of mempool and the default figure is 14 days but remember mempool operators can customize it to longer or shorter than the default value (14 days).

Transaction expiration
Bitcoin Core Config Generator (compatible with Bitcoin Core 25.0)
Mempool expiration: 336 hours = 14 days.

maxmempoolexpiry usually is set as 336 hours by most of mempools.