Upon successful completion of the registration process, the server MUST send,
in this order, the RPL_WELCOME (001) [...] The server SHOULD then respond as
though the client sent the LUSERS command and return the appropriate numerics.
If the user has client modes set on them automatically upon joining the network,
the server SHOULD send the client the RPL_UMODEIS (221) reply or a MODE message
with the client as target, preferably the former. [...] The server MUST then
respond as though the client sent it the MOTD command, i.e. it must send either
the successful Message of the Day numerics or the ERR_NOMOTD (422) numeric.
-- https://modern.ircdocs.horse/#connection-registration
Sable doesn't implement LUSERS yet so it's irrelevant; and the bit about user modes is a mistake, servers always send them after the MOTD.