My question is whether there exists any serious new framework,
guidelines or proposal for writing contracts in a way that avoids
verbal aggression, underlying instead the spirit of collaboration and
commitment that a client/provider agreement should protect.
No. This does not exist.
At the same time, this accepted language seems to indicate that its
author (a lawyer) is more interested in creating the conditions that
would favor a conflict, rather than assisting the parties in avoiding
This is a misunderstanding.
Lawyers prefer one sided contracts that favor their clients. The clarity of these one sided contracts, in fact, greatly reduces the likelihood of conflict.
Even handed contracts are much more prone to conflict than one sided ones, because even handed contracts preserve meaningful rights for both sides that can give both sides legitimate arguments in a particular situation over which they can fight and will fight no knowing how the dispute will be resolved.
I often find the conventional writing style as an instrument to
justify the inclusion of clauses that represent a client/provider
relationship as if it actually were a master/slave one.
This conflates two distinct issues.
Writing style normally refers to matters such as third person v. first and second person writing, sentence length, use of the passive voice, word choice, formatting, headings and the like. There has been a trend towards writing easier to read, "plain English" contracts since the 1980s, although not all attorneys aspire to that style.
For example, most credit card agreements were rewritten in this manner a few decades ago.
Sometimes contracts are written, instead, in a manner that deliberately discourages people from reading and understanding them that also can look more formal and official. These contracts recognize that contract language has a strong symbolic role to play rather than merely a functional, utilitarian role.
Another style issue that comes up internationally is that U.S. contracts tend to be very long and contain all information and rules relevant to the agreement, while in many other countries, lots of detail that would be found in a U.S. contracts is found in the default provisions of statutes or in documents that are incorporated by reference, leaving much shorter, less detailed, contracts.
These style innovations can be found in contracts that are nonetheless very one sided in substance.
A second issue is whether the substantive provisions of a contract are even handed, or strongly favor one side or the other, which seems to be what you are talking about and is not normally considered a "style" issue. There is no meaningful shift of contracts from being one sided to being even handed. If anything, the modern trend is in the opposite direction. Modern contracts tend to be more one sided than comparable contracts were in the past.
The structure of the legal profession means that the default position is that contracts will be initially written in a one sided manner for the lawyer's clients, although sometimes negotiations cause them to end up being more even handed in the end.
There are basically two reasons that even handed contracts are written (and lawyers do know how to write them and do write them now and again):
One is that the parties have comparable economic power (e.g. you see these kinds of contracts between top movie talent and producers of movies).
The other is that it is mandated by some regulatory agency. For example, contracts involving utilities are often set up this way, as are forms authorized for use by non-lawyers by a state agency such as real estate forms.
But, in general, one sided contracts will persist as long as parties are economically unequal.
The way you find out if you have economic power is to insist upon other contract terms and to see what happens. If the other side budges, you have it. If they don't, then you don't.
The extent to which contracts are even handed is just one more economic term of the agreement not unlike, for example, the price term.