This pattern is widely known as “bent four in the corner”. Even though the pattern does not immediately look like that, it can be reduced to such through optimal hypothetical play (in the context of Japanese rules life and death resolution). It is a prototypical example cited in the Sensei’s Library page linked by @Jon_Ko above.
Under Japanese rules, it would be a mistake for either player to play another move in that corner. It is kind of like a seki (but not actually a seki, since White is simply dead) before the game ends. Clearly, any additional move by White is self-atari that just loses all of the stones anyways. Another move by Black at T18 would turn the status of the corner into a ko fight, since it would be followed by White at R19, then Black at S19, and White at T19, starting a ko fight to settle the status.
However, it is a mistake to actually play this out in the game (since Black would need to be able to win the ko fight and waste an additional moving filling in one point of eventual territory to finally settle the ko fight), and Black is not required to actually play this out during the game to assert that it is dead. Instead, life and death status (under Japanese rules) is settled by analysis of a purely hypothetical playout that applies different ko rules. If the players simply resumed the game, it might not be possible for Black to actually win the ko fight, if White has large unremovable ko threats. However, the curious thing about Japanese rules is that White is still dead in this corner, and Black does not have to prove anything by actually continuing the game, even if there are large unremovable ko threats, since the ko rules are actually different when resolving life and death after the players have passed.
I’ve written about this pattern extensively in other forums posts:
Under Chinese rules, the status of this type of shape can be very different, given the global ko threat situation:
This post was adapted from a previous similar post here: A Nakade Question - #8 by yebellz