Very very very low. Bacteria and viruses are normally very sensitive to their hosts, they have a kind of symbiosis that means they can't just arbitrarily infect any species they bump into.
Jumping between species does happen, and when it happens it can be a big problem (see COVID-19, Swine Flu), but there is something like 100 million different virus species out there [1], and only 200 or so are able to infect humans [2]. Despite constant interaction between people and all other species of viral host all over the globe, and millions of brand new new virus exposures daily, jumps are still so rare that they are decade-defining when they happen.
From the conclusions: "... as shown by recent outbreaks of diseases caused by supposed to be extinct microbial pathogens immured in glacial ice for centuries, there is a serious risk for future epidemics (or even pandemics) to happen more often."
I read a book, How to Clone a Mammoth, by ancient DNA researcher Beth Shapiro.
If what she wrote holds true deep in these glaciers (which take a long time to form so they presumably weren't always buried so deep), then the answer to what the risk may be is "very remote". DNA and RNA disintegrates into very small tatters pretty easily, turns out, frustrating the reconstruction of ancient genomes. Bacteria are definitely dead on multiple counts and viruses will be shredded.
