When Han and Beckett first go to the yacht there is a party/gathering.
There are several waiters that seem to have human bodies up to above the mouth, but then the rest of their head is missing and they appear to be robot.
Is there any lore on this? Are they former people who have had their brain removed? Has their consciousness been transferred or replaced with a robot brain?
These characters first appeared in Rogue One and were first identified as the Decraniated in the canon reference guide Rogue One: The Ultimate Visual Guide:
Ghastly medical techniques perfected by a fugitive surgeon known to inhabit the Holy City have produced an order of servants known as the Decraniated. Wounded or incapacitated victims of the ongoing insurgency are transformed with cybernetic technology to become as subservient as droids.
(page 74, Jedha Civilians)
Note that the implication is that Dr. Evazan, who later tried to pick a fight with Luke in Mos Eisley, is behind their design.
The image in the question is also from Rogue One production photography; there doesn't seem to be an entry for them in Solo: The Official Guide.
According to Wookiepedia these characters are called the Decraniated.
The Decraniated were an order of servants produced by a fugitive surgeon Cornelius Evazan, initially for Dryden Vos. Several Decraniated served aboard Vos's yacht First Light. Members of the Decraniated were also seen in Jedha City when the Partisans attacked an Imperial convoy, before the city was destroyed by the Death Star.
you can read the whole entry here https://starwars.fandom.com/wiki/Decraniated
It's worth noting that not everything on Wookiepedia is considered canon. There is an article from screen rant that also talks about these characters. Read it here.
https://screenrant.com/star-wars-decraniated-slaves-explained/
although it offers next to nothing in terms of fresh insight.
