You can run Catalina and Big Sur when it comes out. That said, I would recommend making a carbon clone before updating in the event you are unhappy with how the updated OS runs, as downgrading is a very simple process in this instance. On a 2014 I've kept with El Capitan because it functions very reliably and the heat generated is significantly less than any macOS version that came later, especially when using external displays (regardless of an in-place upgrade or a fresh install). Of course, this has some risks in regard to security, as it is not a supported OS, so definitely worth weighing the pros and cons. As of right now, I personally consider Mojave to be a more refined OS than Big Sur or Catalina, all three of which I am currently using. That's just my opinion though, and this take will vary by individual.
I agree with Mojave offering the sweet spot of stability, compatibility and functions (32bit, Dashboard, etc.)
Newer ist not always better. I keep Mojave both on my 2015 rMBP 15" as well as 2017 iMac 5k. Not any single issue.