I can’t find the api corresponding to CreateMACCommandQueueItemRequest in the new api package - github.com/chirpstack/chirpstack/api/go/v4/api - Go Packages
Yes, it has been removed. The ChirpStack v3 mac-command API was implemented a long time ago, but it probably doesn’t do what you are expecting. Mac-commands should be driven by ChirpStack (based on your configuration and ADR algorithms), not by external applications. The problem is that ChirpStack keeps a snapshot of each device configuration and if you are going to send your own mac-commands, then in the worst case the device could become disconnected.
E.g. if ChirpStack has RX1 delay = 1 second in its database and you are sending a mac-command to change this to 2 seconds, then your device will no longer work as ChirpStack will use the wrong RX1 delay (1 second).