In this answer it is stated that:
If the fuel fully decomposes then return to Earth is still possible but only via ballistic descent
I find it a little bit surprising, but maybe there is enough Δv in the cold gas thrusters?
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Most thrusters on the Soyuz-MS spacecraft use Dinitrogen Tetroxide1 (N2O4, oxidizer) and Unsymmetrical Dimethylhydrazine2 (UDMH, fuel). These are stored (and burned) on the service module and both are fairly stable (from a storage-lifetime standpoint). For safety reasons it was decided that the descent module would use Hydrogen Peroxide3 (H2O2) monopropellent4 as its fuel. Most operations including orientation while on-orbit as well as the deorbit burn are carried out using the service module's engines (N2O4/UDMH); the only thing that the peroxide engines in the descent module are used for is controlling the module's attitude and thus its flight path in the atmosphere5.
1 Nasty stuff
2 Even worse
3 Not that nice either. However, the product of its combustion is water and oxygen which is also what it degrades to over time. This means that any leaks into the crewed descent module wouldn't be too bad.
4 The peroxide spontaneous combusts on contact with a catalyst (platinum for Soyuz-MS IIRC). This typically makes the thrusters more reliable since only the fuel valve needs to work for the thruster to function.
5 The descent module's trajectory can be changed by adjusting the heat shield's angle of attack against the atmosphere. Said adjustment is done with the peroxide thrusters. If the thrusters are not available then the angle of attack can't be controlled and the reëntry will be a bit rough (up to 8.2G rough).
More technical details can be found on this page and on Wikipedia. Also somewhat apropos is this video.
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1$\begingroup$ @uhoh I think there's a good follow-up question there... $\endgroup$ Commented Dec 26, 2018 at 11:15
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$\begingroup$ I see it, thanks! I'll return later, if it's still unanswered I'll see if I can post some of that there. $\endgroup$– uhohCommented Dec 26, 2018 at 11:40
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$\begingroup$ "a bit rough (up to 8.2G rough)" - :) sounds kind of like an understatement. :) $\endgroup$ Commented Dec 27, 2018 at 10:31
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1$\begingroup$ @The_Sympathizer: Perfectly survivable, moderate injuries expected, that's a typical Russian safety feature design. $\endgroup$– SF.Commented Dec 27, 2018 at 21:32
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$\begingroup$ But still not a happy time for the crew. $\endgroup$ Commented Dec 27, 2018 at 21:40