"One way to know if your country is allowed or not is to open https://nitrogensports.eu/ on your browser and if the country is restricted it will give you an "Error 1009 Access denied" message. So if you are living in USA, just open the site and see if its lets you in or not.
Secondly, i have searched the terms and condition page (https://nitrogensports.eu/n/terms) but i did not find any such information regarding banned countries by nitrogen sports."

My partner placed many bets from Costa Rica, with a CR IP. So I don't think this works. CR isn't mentioned in their TC. Have you ever seen a country blocked by this?

This is a lose-lose situation for them. If they say US players are not allowed, 90% of their players realize their money is at risk. If they say US players are allowed, they have to explain how a rule that should be similar to US and Costa Rica allows them to confiscate CR balances, but not US.

When every possible reply is bad, you simply ignore it and hope it goes away.

They are not licensed in a single State in the US. I challenge them to identify a single state where someone can play with Nitro, and not violate their rules. If they can't, they can choose to use this rule against every player when they choose.

This doesn't mean they will steal from you. They reserve that for people that are up a lot. But any book that relies on a "gotcha" rule to selectively steal has much bigger problems going on.

My partner had 10k confiscated by them.

Nitrogen Sports has an ambiguous rule: you warranty that it's legal for you to bet online where you are. It's not legal to bet online at Nitrogen in the US or in Costa Rica. They used this rule to take 10k out of an account after the player was active for 4 months. So they know where a player is playing, and selectively enforce it if they are sure you are a long-term winner.

This same rule lets them confiscate any balance from a US player. When a book starts confiscating balances on ambiguous rules, there are much worse problems going on. If you play and win here consistently, I wouldn't expect to be paid my winnings.

FYI I'm Elihu Feustel, same as in the twitter thread on this topic:
https://twitter.com/d_feustel/status/1521178127036305415

This doesn't make sense. You ask for a payout. 9 hours later, no payout. You post. Still no payout. Then 40 minutes later, your balance is put into action? And Stakebtc responds two two minutes after that, explaining that your balance is all pending?

Ignore any fraud or IP hacking claims. Why wasn't the payout made for 10 hours?