From what I can tell, the dress for altar servers, deacons, and priests has had lots of variation over the years. And it still does.
In the parish where I grew up all altar servers wore black cassocks all the time. I’m pretty sure that must have been all we had. Back then, seminarians, deacons, and priests all wore roman collars, (at least when in public.) Of course there were no permanent deacons then.
Now the seminarians no longer wear attire that clergy wear. Outside of liturgy, permanent deacons “on duty” usually wear deacon polo shirts. Or they might wear a deacon pin on the collar of a regular shirt. I have now idea if transition deacons have any special rules regarding dress. The dress for altar servers varies from parish to parish. Those at my parish wear brown robes.