There have been many proposals to replace 'Master' with 'Controller'; whilst this is clearly an honest attempt to provide a neutral alternative it reflects the proposer's lack of understanding, probably because they don't have the lived experiences of millions of British people, of how vile this term this actually is.
In the UK many generations were brought up on a popular animated children’s TV programme called 'Thomas the tank engine', narrated by the Beatles drummer, Ringo Starr, and later by Michael Angelis, sadly recently deceased. After the steam and diesel railway engines, the main character was known as 'the fat controller'. Obviously, in the previous century peoples' feelings were irrelevant but in this new age of enlightenment we can now understand how insensitive such monikers were.
The term 'fat' may be an accurate descriptive term for a model character in the upper quartile bodyweights of model character populations but the problem arises because the controller was almost always angry - mostly about trains that didn't run on time. The effect was that young impressionable minds were inculcated with the certain knowledge that fat model characters, and by extension fat real people, all have negatively connotated anger management issues.
Fortunately the US viewers were spared this anti-fat hatefulness because someone, properly educated in sensitivity appreciation studies, picked up on the problem and demanded changes to ensure the character was only ever referred to by his proper title Sir Topham Hatt. Even though the US version of the controller isn’t depicted as fat (which presumably is upsetting to fat people as being unworthy of being controllers on US TV) many will make the connection with the original, offensive version. Knighted gentry (those invested with the title ‘Sir’ by the British queen), trawling through old internet postings will discover and be upset by the implication that their peer group have a tendency to irate personality disorders.
This leaves many millions of Britons and probably many citizens worldwide of nations who bought the rights to the original, unexacerbated version, scarred for life with strong associations between the word 'controller' and fatists with their offensive, morally repugnant viewpoints. You can't use 'Director' as a euphemism either as the fat controller was described as the fat director in the original books on which the TV series was based. You might get away with it though, for awhile at least, as that isn't widely known.
And in case someone doesn't get it, the above is written with tongue in cheek - please don't claim that I'm trying to suggest a moral equivalency between human slavery and fatism. Please understand my viewpoint – almost every word will have negative associations for somebody.