This article details the error seen after upgrade.

The EX4400 device was upgraded from 23.4R2-S4.11 to 24.4R1.9 and then gave the error:

Command [set system services ssh hostkey-algorithm no-ssh-dss ssh-rsa] from the additional commands has an error in hostkey-algorithm: syntax error

The "hostkey-algorithm" command has been removed from Junos release 24.2R1. Starting from Junos 22.3R1, the "hostkey-algorithm-list" option has been introduced, which is why the syntax error is being observed after upgrade to 24.4R1.9.

Starting in Junos OS Release 22.3R1, we’ve introduced the hostkey-algorithm-list statement at the [edit system services ssh] hierarchy level. This enhancement enables you to configure only the specified SSH hostkey algorithms. The system automatically disables the remaining unspecified hostkey algorithms. In earlier releases, you need to disable the hostkey algorithms explicitly. All the hostkey algorithms at this hierarchy enabled by default. The DSS algorithm is no longer available at this new hierarchy. In addition, we've deprecated the hostkey-algorithm statement at the [edit system services ssh] hierarchy level.

Starting in Junos OS Release 24.2R1, the hostkey-algorithm statement at the [edit system services ssh] hierarchy level is removed and can no longer be used.

https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/system-edit-ssh-host-key-algorithm.html