Not to turn this into a semantics course, but let's not forget the difference between a check and a counter.
A Kyurem counter would be able to switch into it manually and still defeat it, even behind its Substitute. A Pokémon like Slowking would not qualify, because it would come in to Future Sight the Sub then switch to another Pokémon that can handle Kyurem after Future Sight breaks the Sub. This makes Slowking a Kyurem check.
The list of actual Kyurem counters is extremely limited. There are more offensive and defensive checks, but they would either need to be given a free switch into it or require that no Substitute be up on the other side of the field.
Also, another thing worth noting is that, while a Pokémon like Bronzong counters Kyurem, its limited viability means that it would not be worth running in most cases, whereas Clefable and Mega Scizor are both reliable and viable.
Encore Alolan Ninetales is also able to switch into Kyurem and lock it into a move, but its usage is exclusive to Veil teams.
Bulky Volcarona is a good switch-in as well and, like Mega Gardevoir (which saw some usage on Silver server), can hit Kyurem behind its Substitute. However, Volcarona is weak to Stealth Rock, making it a less reliable answer than Clefable and Mega Scizor.
Also, for those of you who enjoy looking at statistics, here are the Kyurem numbers for Silver server and for Gold server that were shared in the official PRO Discord by Wally. While statistics are indicative, they still show that Kyurem excels with certain teammates that can handle some of its weaknesses.
I would also encourage each of you, regardless of your opinion on Kyurem and whether you think it is broken or not, to avoid looking at match-ups in a vacuum. Kyurem is not isolated, it is paired up with Pokémon that can help it to handle its checks. Just like you, the Kyurem user can also switch out and not be locked behind Substitute. There are many variables to consider.