They're called eternity clauses and they're not uncommon in constitutions. The Constitute Project has a list of
75 constitutions with 'unamendable provisions' in its database (this may not be entirely accurate given how they coded it, but whatever). Besides Germany, which is probably the most famous example of an eternity clause, of major democracies, Brazil, Greece and Portugal have eternity clauses which protect some fundamental rights. The French Constitution of 1958 (and before that the 1946 constitution and the constitutional laws of the Third Republic following the amendments of August 1884) states, in article 89, that the "republican form of government shall not be the object of any amendment", although article 89 itself is not unamendable. The Italian constitution (and probably some others) prevents any amendments to the republican form of government.
Eternity clauses banning any modifications to the bans on presidential reelection didn't prevent authoritarian presidents in Honduras and El Salvador from getting reelected anyway, thanks to lackeys on the courts coming up with creative interpretations of the constitution. It seems like Egypt's 2014 constitution also blocked amendments to provisions on presidential reelection, amusingly.
Dictatorships can also use eternity clauses to entrench the regime: the Cuban and Iranian constitutions both have eternity clauses which effectively prevent any amendments to the forms of government.
Some constitutions also have entrenched clauses which requires special majorities for amendments to certain provisions. I guess the most famous one would be the unanimous consent formula (s. 41) for amending the Canadian Constitution, which requires the unanimous consent of federal Parliament and each provincial legislature for any amendments to the monarchy, bilingualism, the Supreme Court's makeup, the 'senatorial clause' for provincial representation in the House of Commons (no province has fewer seats than it does in the Senate) and s. 41 itself. In 2014, the Supreme Court, answering a reference question from the Harper government on Senate reform, also held that abolishing the Senate would require unanimous consent. In actual Canadian practice (the unwillingness of federal governments to undertake serious constitutional reforms and the near-impossibility of obtaining unanimous consent from all provinces), s. 41 is basically an eternity clause.
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