That is the big elephant in the room after this deal. The thing is: no one can tell by now.
From a technical point of view, Jolla's licensing model allows it's partners to access the SFOS source code, make adjustments and then use the resulting product. Some of those improvements can be backported into SFOS. So it is possible to implement backdoors unnoticed since a big part of SFOS (Silica) is not open source, meaning you can't review new changes for such implementations.
On the other hand, there is a big difference between Jolla Oy (which develops SFOS) and the Jolla Ltd which is the business construct for licensing SFOS and from which Russia Telecom now owns the majority of shares. If they get any influence (and if they do, how much of it) over the SFOS development with this deal is hard to tell. Only Jolla can tell, but probably won't. That's why there are all those wild speculations here.

Let's look at that matter from the perspective of motivation:
The first licensing deals in Russia for SFOS were to obtain an independent mobile operation system for governmental use which is not associated with the USA or China. With this motivation, you're target probably isn't to implement backdoors, but to make the system more resilient and safe to be protected against foreign intelligence services.
Now with the Russia Telecom deal and the focus towards consumer market, this changes. Seeing Russia's tendencies towards internet censorship and crackdown on privacy-friendly tools, I'd say there is a fair chance for adding backdoors as surveillance measures. This would probably affect the russian SFOS at first, but unfortunately it's not infeasible to find it's way back into SFOS itself (and be it unintended when someone forgets to remove it from a backport).
Again, only Jolla can tell about possibilities or countermeasures (if they are not bound by some kind of NDA). So in the end, all we can do now is wildly speculating and maybe look for (in-existent) alternatives.
From my point of view, the best way for Jolla to counter all those wild speculations and fears about russian influence would be to finally move all SFOS components to an open source license, so everybody can check the changes.

One final thought: We are arguing here about possible privacy violating implementations to our operation system in the future while it contains the unaudited closed source security sh*thole called Broadcom baseband from the very beginning.