Discussion: https://www.phpbb.com/community/viewtopic.php?t=2654422

As per the Redis docs https://redis.io/docs/latest/commands/expire/#appendix-redis-expires there's no default TTL, also TTL of 0 is not supported either.

If 0 is set for TTL, Redis cached key just never expires and lives forever until got deleted.

Additionally, another Redis doc https://redis.io/docs/latest/commands/set/ mentions that "Since the SET command options can replace SETNX, SETEX, PSETEX, GETSET, it is possible that in future versions of Redis these commands will be deprecated and finally removed."