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I tried the following:
MariaDB [test]> SET @@global.sql_mode := LAST_VALUE(@@session.sql_mode := 'STRICT_ALL_TABLES'); ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ':= 'STRICT_ALL_TABLES')' at line 1
Not surprising: after all setting @@session.sql_mode requires SET. But this is something one might wish to do.
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If LAST_VALUE() worked with server variables, that statement would assign the same string to both session sql_mode and global sql_mode.
Server variables can only be assigned in the SET statement. Not in an expression. User variables can be assigned inside of an expression — that's why they work in the LAST_VALUE, and often are used in the SELECT statement. But you cannot use server variables this way (not in the SELECT statement either). I don't think we should make an exception for "LAST_VALUE in the SET statement" use case. So what you're really asking about is to allow server variables to be assigned inside an expression. This looks like a reasonable feature request.
But I'll close this MDEV-6176, because we cannot implement it the way it's formulated (allow server variables to be assigned in LAST_VALUE in the SET statement). And it's too specific to be converted to a more general one (allow server variables to be assigned inside an expression). Feel free to create a new feature request for server variables in an expression, if you'd like it to be considered.
What did that statement suppose to do?