I run a gritty fantasy D&D where death is on the table. Even routine combats can end in PC deaths if the PCs are weakened, or just have really bad luck. I enjoy the tension you get in games when there are stakes on the table - I find it focuses the minds of the players and inspires them to think more creatively rather than sleepwalking from combat to combat and employing SOP tactics.
One important consideration with this approach, though, is you need to make it clear to the players that scouting, evasion, and retreat are important tactics. They shouldn't assume that every encounter is calibrated to be defeated in a standup fight. I find once players recognize that they have a lot of control in where and when or even if they engage in combat, they more readily accept the potential for PC deaths and TPKs.
One important consideration with this approach, though, is you need to make it clear to the players that scouting, evasion, and retreat are important tactics. They shouldn't assume that every encounter is calibrated to be defeated in a standup fight. I find once players recognize that they have a lot of control in where and when or even if they engage in combat, they more readily accept the potential for PC deaths and TPKs.