The Bitnodes data show that in 2018 the number of reachable nodes in the Bitcoin network decreased by 15.5%. At the time of writing, the number was 10,004. During the same period, the number of unreachable nodes also decreased by 33%, from 98,000 to 65,500, according to CCN.
A reachable bitcoin node is a node that receives and sends connection requests from the network, and also keeps records of all transactions. At the same time, the unreachable node only establishes outgoing connections, but does not accept incoming ones.
Reducing the number of reachable nodes is fraught with increased centralization of the Bitcoin network and in the future may lead to an attack of 51%. According to the publication, the reduction in nodes number is associated with the miners which stopped operation due to a sharp drop in profits.