I thought that you are able to customise the automation (optimisation plans). Does steam DF have that possibility?
It is different. If I understood correctly, DF chooses the best available dwarf (according to skill) at the time the job is processed. The optimizer chooses the best dwarves (according to role ratings) globally to fill labors according the plan. DF does not need a plan because the job pool will determine how many dwarves are needed. But the "available dwarf" part may be the main drawback of this method from what I have read around, since the game won't wait even a little for a much better dwarf to become available. Other than that, I think the main difference is "skill vs roles".
Beyond that how much effort would it be to keep it?
Right now, very little. On the long term, it is a question I need to ask for a lot of features. I am still unsure of where DT is going.
Dwarf Therapist was first created for assigning labors. But, in my opinion, this is now obsolete. Today, I would rather define DT as an overview for your units (citizens/visitors/livestock) helping you finding the right one for an arbitrary purpose, and occasionally apply the change once you found it. "occasionally" because right now what you can change is very limited (labors, squads, livestock flags). Jobs may not only be decided through labors but also assignment to workshops, building occupations, noble positions, ... (I'm also thinking of Putnam who said that medical labors should be replaced with assignment to an hospital, I can imagine occupations becoming more important in the future)
There is also the technical side. The way DT interacts with DF is very messy and does not allow for complex changes to the game data. This needs to be replaced so DT can evolve.
Together this leads to a lot of changes and it is a bit overwhelming. It is also a good occasion for rethinking DT purpose, and try cleaning some cruft (I'm also considering restarting from scratch).
Back to the initial question, my motivation for working on features I may throw away just after is not very high.
In the case of the labor optimizer, it is also a feature that feels a lot like an addon, it does not really fit with the rest. If it was not for its use of role ratings, I'd say it would be better as its own DFHack plugin.