Can I file for divorce if we’ve lived apart for years but never formalised a separation?

Short answer

Yes, informal separation, once it has lasted three years with your spouse's consent or four years without your spouse's consent, can be used as one of the six facts to prove your marriage has broken down irretrievably. It is not mandatory to produce a signed Deed of Separation to rely on this fact; what matters is being able to prove, through evidence such as different IC addresses or witness accounts, that you have genuinely been living apart for the required period.