Unless I missed something at the time, EU residents (not residing in the UK) who were not EU citizens never had an automatic right of entry to or right to remain in the UK by virtue of EU residency (with some very narrow exceptions like accompanying an EU citizen spouse - though the UK made exercising this right very difficult in practice so that it was usually easier just to apply for a visa).
Much as I thoroughly oppose Brexit it's worth pointing out that other EU countries also made excersing the right to travel with a spouse difficult.
Before Brexit I gave up trying to take my wife on a couple of business trips in the EU as they were too short notice to arrange the supposedly free visa, which in theory shouldn't have been even needed but definitely was in practice.
Nothing to do with science, but one exception is school children. It used to be easy for a class of children from France to visit the UK with a special group passport for school/youth groups, which could include the Syrian refugee child etc. That's no longer possible, so these trips (to help learn English) now mostly go to Ireland.
