Some constitutions contain parts that can't be changed. The most famous example is probably Germany's basic law (technically not a constitution) where article 79(3) prevents changes to articles 1 or 20 of the basic law [1]. Wikipedia lists some other examples [2]. I'm not aware of anyone setting labor law in stone though, that would seem foolish.

1: https://www.gesetze-im-internet.de/englisch_gg/englisch_gg.h...

2: https://en.wikipedia.org/wiki/Entrenched_clause

Probably not the kind of law you were thinking of, but articles 1 (protecting human dignity) and 20 (enshrining the principles of democracy and rule of law) of the German constitution can not be changed or repealed.