streaming replication and wal 's usages

HI List

I am running PG 10.2 with streaming replication, no archiving option
with 10 slots replication option , I had the slave down for 6 hours 
which generated 56GB of wal files in pg_wal dir,  after slave was brougt
online and catchup with master , the wal dir was still growing on slave
with more than 25GB where in archive_status where all the files where in
*.done files , eventually the /wal directory reached 100% but still no
wal files removed from archive_status

I am trying to understand what I am missing in this confg ( the master
and slave are cfg for pg_dump backups the switch-over is done with
pg_basebackup using repmgr4.2.last release )

the present db size 140GB OLTP type transactions , with an avg 10-12GB
new data  growth per day.

Here is my cfg

wal_keep_segments = 300
max_replication_slots=10
wal_log_hints = on
checkpoint_timeout = 5min
max_wal_size = 1GB
min_wal_size = 80MB
archive_mode=off

hot_standby =on

Thank you

Isabella