a lot of formerly toxic sites are built different. instead of developer building and selling or leasing 5-10 years, they build and lease for long term 25+ years so any issues with the toxicity are pre negotiated with the developer, builder, tenant. and everyone is covered financially so if actually toxic in year 26, developer can walk away risk free

edit: this is going on in philly right now with hillco developers on former gas facility that closed after explosions