There are two kinds of unforgivable sins. The first is where the person cannot be brought to repentance: no repentance = no forgiveness.
It is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt
There’s another category of sin for which there is no forgiveness: Blasphemy against the Holy Spirit. Even then, Jesus and the disciples tried to bring the Jewish leaders to repentance and sought their forgiveness (Acts 7:60; Rom 9:1-5; Luke 23:34).
So while there are some sins that are unforgivable, and we have some general idea of what their shape is, we should never think that it’s too late for someone — either ourselves or someone else — to turn back to God and seek forgiveness and grace.