Is it normal for a department not to specify where a PhD candidate's funding will come from before the candidate accepts the position? I've been accepted into a Philosophy PhD program, and they have told me that I will probably have a job as either a teaching assistant or research assistant for each of the next five years and that there's a decent chance I'll get some kind of summer fellowship for one year. However, they won't tell me which years I'll be a TA and which an RA, nor will they guarantee my funding for any time period.

I'm curious whether this is the norm, both in Philosophy and in PhD programs more generally.
It is certainly not unusual to not specify where funding will come from, even when funding is guaranteed. Often, funding sources aren't obtained until immediately before the funds are used.

Research assistantships are probably the most predictable, because they can come from grants that last for multiple years. However, even long-term grants may not perfectly overlap with the timing of a graduate career and can shift from year to year. Project assistantships are similar, though they may also have similarities to teaching assistantships.

Fellowships, traineeships and other types of scholarship are probably the least predictable. They are often competitive, short-term, and although some may be within a given department (so the department knows it has 2 fellowships to award, for example) they still cannot promise them for a specific student until the competitive process is complete. An exception would be a traineeship/fellowship that is offered to a student for their first year, but funding after that is less concrete.

Teaching assistantships are often a way to make up the difference and to fund students who can't be funded another way, although in some cases departments may require students to fill TAships to fill a need. In my experience, when a department guarantees funding they only mean that they promise you they will find you a TAship (or PAship) if they cannot fund you with other means or you cannot transfer to a different professor who has RA money.

For your situation, the most important information you have is that your funding is not guaranteed. In my experience at large R1 universities in the US, it is the norm to not have guaranteed funding in some disciplines (humanities, social science) and to have guaranteed funding in others (engineering, science) - it mostly depends on the availability of research funds. I would suggest that senior students in the program you are applying to may be your best source of information: if you have an in-person interview where you can meet students, you should ask them what their funding situations were like, or if you already interviewed, hopefully you met with some students that you can ask. The program may also have some statistics, but otherwise you have to base your decision on a fairly vague non-promise.