Does the Planck Spacecraft end the question of sterile neutrinos? The 2015 results of the Planck satellite only found evidence of 3 neutrino's with the 4th (sterile neutrino) unlikely.
However, in May 2018, physicists of the MiniBooNE experiment reported a stronger neutrino oscillation signal than expected, a possible hint of sterile neutrinos.
So in 2019 are we still looking for evidence of a sterile neutrino? or is it no longer thought to exist?
 A: This is an open research question, so there's no definitive answer.
Firstly, sterile neutrinos are required by a very favourable mass mechanism for the neutrino, which does not violate the SU(2) electroweak  symmetry (it's this symmetry which makes the neutrino massless in the bare Standard Model). The mechanism is called the see-saw mechanism, and postulates the neutrino is a Majorana fermion (it is its own anti-particle, like the photon except that the photon is a boson). If the neutrino is its own anti-particle, we should observe a process called neutrinoless double-beta decay. That would be very strong evidence that the see-saw mechanism is the correct mass mechanism, and would provide very strong evidence for the sterile neutrino. However, it has not been observed yet.
Secondly, the anomaly found in a host of nuclear-reactor experiments, but most prominently in LNSD and MiniBooNe has to be taken with a grain of salt. The nuclear-reactor experiments are not very clean, and the LNSD & MiniBooNe results are actually in tension with one another. The observed anomalies actually occur in different neutrino-energy ranges. I went to a talk about MiniBooNe, and they have to model some systematic error in missing pions which, when extrapolated to lower energies, becomes extremely large. See Fig. 1 in https://arxiv.org/abs/1805.12028. The large red bars are misidentified pions. The value for that is not measured, but has to be simulated. That might cause large amounts of systematic error not accounted for. Furthermore, the anomaly could be explained by unknown pion physics.
Regarding the Planck data, you also have to be wary that this is a cosmological measurement. Such a measurement would (probably, but I don't know) rely on 1. how different particles contribute to the energy density of the universe 2. how additional particles change particle-physics processes in the early universe, which would effect what we observe. Their conclusions would be extremely model-dependent, and should not be taken as conclusive.
I would say that the see-saw mechanism is a very favourable mass mechanism for the neutrino, and the hunt is still on to discover whether 1. the neutrino is a Majorana 2. whether there is a (right-handed) sterile neutrino.
