Non-Russians often assume some sinister motive behind actions like that, but reality is much simpler.
The authorities failed to shut Telegram down while making active use of it themselves. So now they are giving up because there’s not much point in continuing the efforts.
By doing so they would send the wrong message though "feel free to use it against us; we're neither closing nor monitoring it", which would be like shooting in their feet. Unless they're just pretending it.
Every government fears what it cannot control, especially communications networks that could be used to subvert the status quo; this applies to all governments in all countries, including so called democratic ones.
To me they're still actively monitoring it, and I'm 100% sure they have the means of shutting it down just in case of real necessity.
On the other hand: if you force people to use VPNs to use Telegram ... you're forcing people to learn to use VPNs.
And if you're afraid of people talking about stuff without you knowing, the last thing you want it large parts of the population (or at least those parts you're interested in) using more encryption. Basically: you can't stop it, trying to stop it has negative side-effects, so stop trying.
I’m sure they are still actively monitoring, but I don’t think they have the means to surgically disable Telegram without shutting down all of internet in Russia. They’ve been trying unsuccessfully for the past few years.
The authorities failed to shut Telegram down while making active use of it themselves. So now they are giving up because there’s not much point in continuing the efforts.