Should this type of questions be on topic?
Why they shouldn't? : They are not exactly about Islamic teachings and/or practices.
Why they might? : Somewhat related to Islam.
What do you think?
Like I said with the Islamic history, if it can be answered with a hadith, it's should be on topic. Especially since much of the documentation on the sahaba were via hadith. People around here usually have a good grasp of hadith, which means that you can get a lot of content here, compared to from historians (which usually end up taking from hadith anyway).
The sahabah tend to be held in high-esteem by most (if not all) Sunni Muslims. The same could also be said of the Shi'a, even though they may differ on which of the sahabah warrant such respect.
These were the people who were closest to the prophet during his ministry. Among all the Muslims in the world, throughout history, these were the ones who were (theoretically) best able to understand the prophet's true message and apply it to their day-to-day lives. As such, the more that one knows about the lives of the sahabah, the more one knows about being a good and proper Muslim.
This principle is especially prevalent among movements such as the Salafi, which consider emulating the sahabah (as well as the earliest generations of Muslims) to be almost as important as following the sunnah of the prophet himself.
In addition to that, knowing about individual people (including sahabah) can be of vital importance to the sciences of hadith; without knowing whether any (and every) particular person in a chain of narration is reliable, it is impossible to determine the authenticity of the narration itself. In-depth studies of the lives and characters of everyone involved is necessary to determine reliability.
Factors such as "Has he ever been known to lie or forget things," and "Was he ever even in the same city as the person he's allegedly narrating from," can and do differentiate a sahih hadith from a da'if one.
Although Sunni Muslims (as far as I know) tend to consider all sahabah as reliable narrators, I'm not sure if this is universally accepted among all schools. This would, however, definitely apply to those earliest generations of Muslims who were involved in narrating hadiths, but who never directly met the prophet himself.
While the above situations may not make every question about the sahabah on-topic, they do cover a lot of ground between them. Rather than trying to define a general line between exactly which sahabah questions are on-topic and which aren't, it's probably best to say that such questions are acceptable unless they're very clearly not.
In other words, they should be judged on a case-by-case basis.
As for the question linked in the opening post, it is specifically mentioned that Talhah is one of the Ashara al-Mubasharin bi-l-Janna. A strong argument can be made that knowing how he lived, and died, is a good way to know how to attain paradise for oneself.
And if striving to attain paradise is not "about Islamic teachings and/or practices," I'm not sure what is.