Hmmm...I'm not sure I'd call it
slavery as such. I should state right away that I'm no authority on legal definitions, so don't expect a meaningful answer, but I'll try to articulate my view of the Jem'Hadar. They're victims, certainly, and repressed. Their treatment might be comparable morally to slavery, in many ways- Jem'Hadar have no choice but to serve the Dominion as warriors- but I'm not sure if they're slaves in the sense of being owned legally by another. They're kept deliberately
dependent on another- they need their White- and are subject to the whims of the Founders. They are certainly controlled and refused the opportunity to leave or make their own path in life, but I'm not sure if being part of a super-restrictive repressive state constitutes being a slave in any technical sense. We might well call it "slavery", but it's more like being subjects of a very, very overbearing and controlling leader, who places limits on every possible freedom. Is it written anywhere in the Founder's laws (and I don't think they would even
have any written laws or documents outlining the nature of the Dominion) that the Jem'Hadar belong to the Founders? The Founders control the Jem'Hadar (albeit indirectly most of the time) but they don't feel the need to justify it by introducing any concept of legal ownership or obligation for Jem'Hadar service. They just control. They don't have to justify it- they're the Founders, the creators, and they act as essentially one multi-faceted being, through the Link.
I suppose we might say the Jem'Hadar are very similiar to slaves in that they
are taught that they're obliged to serve another and be "owned" by another- in their case, this "reality" is forced on them not through legal means but by through their genetic programming, which directs them to consider Founders their superiors and to obey a Founder. We know elder Jem'Hadar can exercise their free will and begin to question those directives if free of White, so in that sense the Founders are attempting to force on them a role that might be at odds with the individual's capacity for self-determination and subordinate them by "natural law" to control of another being. But still, you could point to many parallels in real life that do not legally constitute slavery, I believe, whatever we may morally think about the comparison.
If the Jem'Hadar are slaves, are all the Dominion subjects slaves? Or is the fact that often those subject races are left more or less to their own devices (e.g. the Karemma) a point of disparity? Jem'Hadar are subject to far stricter controls, on a genetic level, for example, despite serving as part of the Dominion's state apparatus. A Jem'Hadar would be higher in the system than a Karemma, because he enforces the law of the Founders, but he is also subject to greater repression.
The way I see it, the Founders don't have slaves in the Jem'Hadar, they have children- abused and maltreated children. The relationship isn't a matter of legality, it goes deeper than that and becomes more complicated. Jem'Hadar are children who do as they're told and who have been "programmed" on a genetic level to serve a particular role for the glory and protection of the "family". Founders created the Jem'Hadar, so in a sense they have a responsibility for their development. I've always seen the Jem'Hadar (and anyone who knows my general stance on human culture and history will have seen this coming a mile off

) as being a (exaggerated of course) stand-in for how many human cultures condition their sons into warriors, training them from birth for one essential purpose- fighting for the protection/glory/whatever of the tribe or nation. It's repressive, it's controlling, but it's not really slavery in most cases, because it isn't really
legal control these nations and elders have over the young males, but something deeper and far less easy to define or indeed attack. Oh, it might well become legal control
at times, but that's actually often unnecessary- for all people like me moan about mandatory service, it's existence is actually a sign that the youths there are on the whole far
less controlled and indoctrinated. To me, Jem'Hadar are on a genetic level what many young human men are on a cultural and psychological level- "programmed" to fight, to restrict themselves to a purpose rather than engage in exploration of their full faculties, and kept loyal through the manipulation of that (genetic/ideological) "code" to prevent self-sufficiency (though the concept of an actual
physiological leash in the form of White, rather than a culturally-installed ideolgical leash is of course taking it to a whole new level). It's important to understand, of course, that most humans wouldn't define this as slavery- it's both more insidious and ten times more complicated. Given my political views and personal opinions, I like to make comparisons to slavery (and as an equivalent to my opinions I will point out we can identify similiar if different forms of ideological control over females in many cultures, and perhaps suggest similar comparisons) but it isn't slavery. Slavery is shallow and simple. Relationships not of a legal nature but of ...well...a human (or general sapient) nature are so much harder to define.
So, I see the Jem'Hadar as connected to the Founders on a level far beyond the legal. The Jem'Hadar are, to my mind, essentially mistreated children of the Founders. The Founders created them, and are essentially their parent race. But they are very poor parents indeed, seeing their "children" as tools or pawns to reinforce their own sense of security, and denying those children full expression and the ability to grow into their true potential. The Jem'Hadar need "liberating", I would agree. But the situation is complicated by the fact that they have been essentially programmed both to ignore any possibilities of self worth disassociated from the narrow role they're given, and to see that narrow role as the very thing giving them worth. Getting them out of that situation would be tricky (as it is with people in reality).
Basically, I see them as maltreated children and/or abused conscripted soldiers. And while those are in many ways very similiar in my mind to slavery, I'm not sure the term truly fits.
Interesting offshoot question: are the
Vorta slaves? More priviliged "overseer" slaves?