Praise be to Allah.
Is kombucha halal?
We have read a great deal about the benefits of kombucha tea; the issue of its benefits should be referred to specialists.
With regard to the amount of alcohol in kombucha tea, it is a very low amount that does not lead to drunkenness, hence drinking it is not haram because of that.
Please see the answer to question no. 60212 in which there is an answer concerning fermented tea.
Purity of water
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
“Everything that is called water is pure and a means of purification, whether some impurity has fallen into it or not, if it is known that it has dissolved into it and disappeared. As for that in which traces of impurity are apparent, it is haram to use it, because it comes under the heading of using something haram.” (Majmu‘ al-Fatawa, 19/236, 237)
He also said:
“Allah has forbidden impure things (khabaith), namely blood, dead meat, pork and so on. If any of these things fall into water etc and disappear, so there is no blood, dead meat or pork left at all, similar to the case where alcohol disappears in a liquid, then the one who drinks it is not drinking khamr.” (Majmu‘ al-Fatawa, 21/501, 502)
Please see also the answer to question no. 33763
Based on that, there does not seem to be anything wrong with drinking kombucha tea.
And Allah knows best.