Isa 66:22-24 and Mark 9:43-48 are using Idiomatic language. Mark 9:43-48 is proof that Gehenna is eternal.
Mark 9:43-48 If your hand causes you to stumble, cut it off. It is better for you to enter life maimed than with two hands to go into hell, where the fire never goes out. [44]b 45And if your foot causes you to stumble, cut it off. It is better for you to enter life crippled than to have two feet and be thrown into hell. [46]c 47And if your eye causes you to stumble, pluck it out. It is better for you to enter the kingdom of God with one eye than to have two eyes and be thrown into hell, 48where “ ‘the worms that eat them do not die, and the fire is not quenched.’
Isa. 66:22-24 "For as the new heavens and the new earth that I make shall remain before me, says the LORD, so shall your offspring and your name remain. 23 From new moon to new moon, and from Sabbath to Sabbath, all flesh shall come to worship before me, declares the LORD. 24 "And they shall go out and look on the dead bodies of the men who have rebelled against me. For their worm shall not die, their fire shall not be quenched, and they shall be an abhorrence to all flesh."
Mark 9:43-48 If your hand causes you to stumble, cut it off. It is better for you to enter life maimed than with two hands to go into hell, where the fire never goes out. [44]b 45And if your foot causes you to stumble, cut it off. It is better for you to enter life crippled than to have two feet and be thrown into hell. [46]c 47And if your eye causes you to stumble, pluck it out. It is better for you to enter the kingdom of God with one eye than to have two eyes and be thrown into hell, 48where “ ‘the worms that eat them do not die, and the fire is not quenched.’
Isa. 66:22-24 "For as the new heavens and the new earth that I make shall remain before me, says the LORD, so shall your offspring and your name remain. 23 From new moon to new moon, and from Sabbath to Sabbath, all flesh shall come to worship before me, declares the LORD. 24 "And they shall go out and look on the dead bodies of the men who have rebelled against me. For their worm shall not die, their fire shall not be quenched, and they shall be an abhorrence to all flesh."