Darth is a title for Sith Lords. If Palpatine/Sidious no longer accepts him as his apprentice, does he still bear the title of Darth?
I would say "no", but it's a conjecture and not an undisputable canon fact.
First of all, it really depends on your point of view
You can view the name/designation as something that someone calls themselves, in isolation. From that point of view, Darth Maul is still Darth if he chooses to say so. However, nobody else is obligated to agree with him and call him that.
You can view the name/designation as part of a specific culture, imposing specific meanings and specific customs. In that case, what matters is whether the desired designation will be accepted by that culture as a whole, or its thought leaders (which, in case of Rule of Two Sith, pretty much amounts to the same small set).
If we take the point of view of "this name is a part of specific tradition/culture and thus should obey its rules", then the answer is "NO"
The basis for that is that in this case it's the Sith Master who typically bestows the name on their apprentice (as evidenced in ROTS dialogue when Sidious names Darth Vader).
Given the circumstances of Darth Maul's upbringing detailed in Maul books (he was brought to Palpatine as a baby by his mother and raised/trained by him), he was most assuredly named by Sidious as well.
This means that the name is connected to and thus contingent on the apprenticeship, within the norms of their culture.
You can work around this rule in two ways:
Kill your Master and become the Master yourself. Then you have the position to name yourself whatever you want.
Become a Sith Master yourself (ala Darth Krayt), again basically granting yourself the right to call yourself anything you want.
As such, that name/title is granted by the Master, and therefor is subject to the continued acceptance of the Master
The Star Wars in 100 Scenes factbook explicitly states that ...
At any one time, only two Sith may use the title of "Darth" - The current master and his or her apprentice
Since Maul is no longer the Master or the chosen apprentice, his title is void.
Sidious directly states that Maul is no longer a Sith, and therefore no longer a true Darth, in the Star Wars: The Clone Wars episode The Lawless:
Darth Sidious: [to Maul] Remember, the first and only reality of the Sith. There can only be two. And you are no longer my apprentice. You have been replaced.
Additionally, the Season 2 finale of Star Wars: Rebels has Maul definitively state that he isn't a Darth anymore.
Maul: Formerly Darth, now just Maul.
He is a Darth until proven otherwise
That is to say that until he is challenged about using the title and loses then he is in fact still a Darth. The whole premise of the Sith is that you have to constantly prove yourself worthy. They value strength more than anything else it's the whole point of the rule of two. Basically since Maul was strong enough to survive being cut in half and thrown down a bottomless (this is a bit of a hyperbole) pit than he still deserves the title. So he remains a Darth until Sidious later kills him and his apprentice and proves him not strong enough. As for Sidious not accepting him as an apprentice, that is only because he believed Maul to be dead and he had another apprentice so Maul was nothing but an obstacle to be removed.