This was already discussed shortly on IRC with @ailin-nemui - and per his request i open this issue on it.

Numeric 489 is currently defined only as ERR_VOICENEEDED in Irssi but is used as an error reply (ERR_SECUREONLYCHAN) for non-secure clients trying to join a secure-only channel on several networks.

Right now 489 will be printed in the channel-window and then (with setting autoclose_windows = on) promptly lost with the closed window. It should probably go to the status-window instead/too, as it does with ban, inviteonly, etc.

For reference:
see 489 (and 480) at https://defs.ircdocs.horse/defs/numerics.html
see 489 at https://github.com/ircd-hybrid/ircd-hybrid/blob/master/src/numeric.c

The mode used for this differs between networks, if you are interested in that rabbithole see: ircv3/ircv3-ideas#45 and ircv3/ircv3-ideas#46

Perhaps it would be best to support a "free text" style numeric. but thats just IMHO and without having looked into how numerics are implemented in Irssi: ":%s 489 %s :%s"