I agree that 0-confirmation transactions were never really secure. However,
many operators had a business model that was based on 0-conf transactions
(e.g. the now defunct betting site Directbet). Therefore something seems
to have changed, because otherwise these operators would still be able
to use 0-conf transactions.

I can´t imagine that this is only due to increased fraud by people attempting to double-spend
a gambling transaction (e.g. double-spend after their bet already lost on Directbet). There has
to be a technical explanation why 0-conf transactions are not widely used anymore
when that was still the case in 2014-2015 (at least at Bitcoin gambling sites).