It is consistent with socialism, but not essentially socialist.
> socialism is about central planning
No, it's not. Central planning may be part of some versions of socialism, though there are serious arguments within socialism that central planning in the strong sense is incompatible with socialism, and instead a degenerate single-capitalist form of capitalism.
Socialism is about common/worker ownership and control of the institutions, means, and proceeds of economic production (each of these is a goal and a means to the others.) UBI, especially when funded from taxes on wealth, property, or income where capital income is not excluded or favored, is a mechanism for common participation in the proceeds of production.
> This is why in the 21st century the most prominent promoters of UBI (sometimes called negative income tax) were libertarians.
I think you mean the 20th Century and right-libertarians, though as you state it jt is, in fact, also true of the 21st Century (but there it is libertarian socialists at the fore, who aren't the libertarians you seem to want to refer to with your “UBI isn’t socialist” line.)
I still contend socialism is about central planning. Common ownership and participation in the means of production via democracy is central planning in practice, since people end up pooling their decision making power.
Representives then implement policies decided by the collective, which all people are subject too.
It's opposed to the libertarian vision, which would be for each person to make more of their own decisions individually, with voting less relevant.
Actual socialists, for the whole history of socialism, disagree, but please, don't let that distract you.
> Common ownership and participation in the means of production via democracy is central planning in practice,
When the common ownership is done through the state which directly controls industry, sure, you get central planning. When it's done through democratic worker control of firms, it is no more central planning than when firms are jointly controlled by their capital owners. And there are lots of variations of socialism besides those two.
> It's opposed to the libertarian vision
The idea that socialism is opposed to the libertarian vision is news to libertarian socialists.
I would call what you are describing communism, not socialism. And it's nothing like any major socialist society in existance, because it's unobtainable in practice.
This is why in the 21st century the most prominent promoters of UBI (sometimes called negative income tax) were libertarians.