What does one mean by "a modern design by the standards of the day"? I don't mean this facetiously — it is worth being very clear about this.

If you mean, "did it conform with the standards of the Soviet Union in the late-1970s/early 1980s" — the answer is clearly, almost tautologically, yes. See my recent post here about the motivating factors that led to its design. The RBMK was one of the two major reactor designs for producing power in the USSR at the time, and was "state of the art" at that point, and was designed by a fleet of entirely competent nuclear engineers.

If the question is, "would any non-Soviet, non-Warsaw Pact country design a reactor like this for civilian use?" the answer is clearly no — it lacked several important safety features that by that point had become standard (like the containment dome). Nations outside of the Soviet sphere of influence had a very different approach to safety culture (and engineering culture), and designed their civilian reactors more conservatively (at greater cost).

If the question is, "would any non-Soviet country design a reactor like this for non-civilian use?" the answer becomes... yes. The RBMK shared several features with military-production reactors (notably Hanford's N-Reactor, but depending on how you divide up what you mean by "features," other ones as well) in the USA and other places. Military-production reactors, which are used to generate plutonium, have tended to have less safety factors (because they are operated on remote reservations, and because their main goal — to rapidly circulate fuel within the reactor — conflicts with some safety features and safer designs) than civilian ones. And as I note in the other post, the RBMK was essentially meant to be a dual-use reactor, a civilian reactor that could be used in a military fashion if you wanted to.

So depending on what you're really asking, the answer is a bit different. I think this is a less useful kind of question than, "why exactly did the USSR design a reactor that to us looks so obviously dangerous in retrospect?" Which is what the linked-to post is about: the RMBK was a very specific design created in and for a very specific national-economic-bureaucratic context. That context produced something that had the potential for being a terrible disaster, and unfortunately it lived up to that potential. The fact that reactors are designed to specific national-etc. contexts is not itself remarkable (all reactors are designed for specific contexts; it's somewhat impossible to imagine what a context-less reactor would look like), but it's the specifics of the context that tell you why the Soviets deployed something that turned out so tragically.