Tombstone eviction

I was reading durable delete and came across this statement. " Tombstones take up RAM and disk space, they never expire and thus can never be evicted". https://www.aerospike.com/docs/guide/durable_deletes.html What does this means ??? Tombstones will grow and never get evicted and expired ???

Tombstones are cleared by the tomb-raider thread when 3 conditions are satisfied 1- there is no previous version of the record still on disk (ie all previously used space by these record versions has been overwritten by new write blocks), 2 - no migrations are going on, 3 - the tombstone has been around beyond its configured minimum life (default is one day - can be changed dynamically).