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The UK was not unique in having “opt-outs” from the EU-other countries have them too:

Denmark has a formal opt-out from the Euro. Several other states (e.g. Sweden) have de facto opt-outs from the Euro - unlike Denmark, EU treaties say they have to join, but their governments have decided to ignore that legal obligation - without consequence, since the treaties failed to provide any enforcement mechanism for it

Denmark and Ireland both have opt-outs from security and justice policies. Ireland’s opt-out permits it to opt-in on a case-by-case basis, Denmark’s doesn’t. Until it was abolished in a 2022 referendum, Denmark also had an opt-out from the EU’s military policy

Poland has a partial opt-out from the EU’s human rights charter

Ireland has an opt-out from Schengen (abolition of border controls). Romania, Bulgaria and Cyprus are parties to Schengen, but haven’t yet implemented it