QuentinGeorge
Legend
I don't think any edition has had precise rules for "expert in conveyancy law!"
Except in 5e, the skill system is so broad that a single skill often covers things that would be 2 or 3 skills in other editions of D&D. If Athletics can cover swimming, climbing and jumping. . .if proficiency with Thieves's Tools can cover picking locks and disarming traps. . .if Arcana can cover both knowledge of magic and other planes of existence. . .if Stealth can cover both hiding and moving silently. . .there can be a single proficiency for the various professional knowledge of a field that isn't covered by other things and might fall under multiple similarly related fields of knowledge.
Except, as I've pointed out repeatedly, there's no option, in the RAW, to learn or gain another set of professional knowledge. The idea that once you've learned your initial trade before your adventuring life, and this is the ONLY profession you can know seems rather limited.
Backgrounds DID emerge into d20 in the 3e era, they were originally from d20 Modern, but they weren't meant to be the ONLY thing a character could do as a trade or occupation, they were packages of background abilities to reflect learning and experiences that happened before the adventuring life.
5e didn't invent the backgrounds system, it just imported it from another source into D&D.
Backgrounds are something I don't mind about 5e at all. I'd often toyed with the idea of introducing them into my 3.5 games by adapting the ones from d20 Modern/Urban Arcana. The idea that someone's background is the ONLY profession they know how to do, and the only one they can ever learn, that is the part that's bothering me as a limitation on the system.
This is another reminder that Farmer's Tools should be on the tools list
One of the complaint that was made in this thread was that a lawyer character built this way would be equally good at estimating the likely outcome of a trial based on precedent (INT+history) and at remembering the list of the Roman emperors, and he'd mechanically need to be a very good haggler (CHA+persuasion) as a side effect of him presenting a case in front of a judge. I think having him be proficient in tne tools of the lawyers resolve that problem (as it narrows the proficiency to a specific field while broadening it to several key "skills").
So, from what people are saying and what I'm reading, maybe the best way to handle more professions in 5e is to expand the list of tool proficiences and be a little more open minded and flexible about what constitutes a toolkit.
So, for my Soldier example, a "Soldier's Tools" which would be things like a uniform, rank insignia, standard or guidon, regulations. . .things a Soldier might be issued or carry other than weapons, armor and camping supplies, and that proficiency with those items would be the game system's way of saying someone would have the core skill-set of a soldier.
Or Farmer's tools of a hoe, shovel, plow, wheelbarrow, seed bag etc. being for a farmer.
Or Lawyer's tools being a collection of law books.
In cases where the profession skill might heavily overlap with an extant skill, say Lawyer and History for something to do with legal history I'd allow advantage on the roll. That gets a little bit trickier to adjudicate for something physical and combat related like Soldier, but even then I can think of examples, like giving advantage on deception checks to pass as a soldier, or advantage on attempts to persuade a soldier of something.