"vacuum energy risks all existence"
These may be clues to the demise of the Oort. "Entropy epoch" implies a period of decline and chaos. "Vacuum energy risks all existence" may imply how the Oort destroyed themselves. Interestingly, the use of the word "entropy" my be a double entendre when you consider the use of vacuum energy. You see, vacuum energy, aka zero point energy is a theorized type of energy taken from background energy from the vacuum of space. Vacuum energy is theorized to be infinite, yet tapping into it may affect the thermodynamic balance of the universe causing the heat death of the universe. In this case, we are using the physics definition of the word entropy as well as the common use of the word. This seems to suggest that the Oort did not simply die off slowly, but destroyed themselves by tapping too much vacuum energy. I may have gotten some of my science mixed up here, but I do believe this is the general basis behind the lore.
P.S. In a PM from @James, the lore of The Oortiverse is based on science / cosmology / creativity. Additionally, the devs will likely not comment directly on topics of lore, but rather allow us to speculate and unravel the lore for ourselves. Consider it an archaeological quest! Happy hunting!!