What is this "mixed" citation style that includes both numbered references and author-date formats?
Speaking about major citation styles that I've seen, I think that it could be the IEEE Citation Style, as your example matches the IEEE Style for electronic references (for non-electronic references, the in-text citation is the same, with the only difference, being related to reference list, is that the year is located at the end of the reference entry).

Alternatively, if it is not the IEEE Style, it might be either a publication-specific style, adopted by a particular journal or other publication outlet, or a hybrid style, manually developed by some authors.

Naturally, speaking about using LaTeX for biblographies, there are a couple of aspects that I'd like to mention. Firstly, according to Mori (2009), the citation style that you are curious about resembles the default reference formatting style for LaTeX. Secondly, if you use (or plan to use) LaTeX for producing your publications, the following sources, in addition to the paper by Mori, might be quite helpful for customizing bibliographic features to required or desired style: this guide by Patrick Daly (note that it describes a quite old version of natbib package - try to find a more up-to-date version or a similar detailed guide), this brief guide by Ki-Joo Kim and this excellent answer by Alan Munn.

References

Mori, L. F. (2009). Managing bibliographies with LaTeX. TUGboat, 30(1). Retrieved from https://www.tug.org/TUGboat/tb30-1/tb94mori.pdf