Not sure if there’s a way to downgrade to whatever firmware you want, you can of course try by grabbing whatever version you want for your Kobo Glo from here:
https://pgaskin.net/KoboStuff/kobofirmware.html
I’m not sure how far back you can go without tinkering. I’m sure if you’re really dead set on it you can go quite far back though you’d likely need to bypass registration (searching the forums should get some instructions as well as Jon telling you and everyone else not to do it because you risk losing the books you couldn’t buy from kobo… don’t think too hard on that it’s not meant to make sense).
Though can I ask why you want to downgrade? Unlike Kindle with Kobo you can patch the firmware easily no jailbreak needed. The Home Screen has changed over the years but at this point I think even the most die hard supporters of the old Home Screen from I think 3.19 and earlier would admit the changes and performance boosts are worth it. And I can’t think of any features that have been removed, maybe setting the font weight and sharpness separately though you can sort of patch for this.
There’s generally no reason not to upgrade because you aren’t actually being restricted in modifying the device.