Delta T50 is what Europe has been using for a while, except in the UK, where we used Delta T60 until 2013. But now we use T50 too.
Delta T50 is based on a much lower expected boiler temperature. Which is more in-line with modern insulation and a fear of global-warming-induced-apocalypse.
Delta T50 has been officially in use for some time now. T60 is still lingering around because this is an industry where change is relatively slow.
Delta T70 is seen a lot less often and is generally only used by certain suppliers. If you’re given a BTU at Delta T70, it’s usually best to convert this into T50 or T60 to make comparing heat outputs much simpler.