Hi Markus!
I wrote down an idea for out-of-band warning messages in MDEV-31839 where by reserving one "system variable" for warnings, we can inject them to the initial OK packet that the client receives. This way clients that understand the server variable changes are able to display them to the users. The MariaDB Connector/C is already able to extract the system variable change information even if it's in the first OK packet sent after authentication. I found this sort of accidentally when I was testing some code for MDEV-31609.
I would not be surprised if after MDEV-31609 this could be implemented by just adding a dummy server_motd global variable that when set is sent to the clients.
The server supports a COMPILATION_COMMENT[1], which is shown in the SQL console for connecting clients. This is already used in the wild to e.g. inform users that an old version of MariaDB is out of support and should no longer be used. If that field could be changed or extended at run-time it could do the job you envision above - which would be useful indeed as a generic way to communicate things to users. [1] https://github.com/MariaDB/server/blob/485c9b1fb320958b13253a49d4480ee9decf9...