UBIs can be a good part of socialism, but not necessarily an essentialist value of it, though it's not as well-utilized under capitalism...

If Feudalism means the rule of Feudal lords, by ownership of the land and thus crop rents, and capitalism means the rule of capitalists, by ownership of capital and thus profit

Then with socialism, it's the rule of society, by communal ownership (state or not) of our industry towards societal goods, such as food, shelter, etc. and avoid the crises that come with it

If you reform the system without changing its system, it will rhyme up its mistakes all over again (do the same action but with worse effect to society)

Btw though: don't most of the ideal Socdem countries, whom you call socialist, in the West rely on exploitative unequal "exchange" , and the Socdem countries of the Global South are slandered and sanctioned, the most extreme example being Venezuela?