Ultimately, the best solution for me was to turn off Auto-Correction. It will still suggest “2.0” but will not replace 20 with it.
Turning off Predictive text didn’t work. Resetting the dictionary didn’t work. Using Text Replacement to set 20 as the shortcut for 20 didn’t work. With all of these, if you kept typing after 20, 2.0 would still replace it.
I tried disabling most of the keyboard options and then enabling them one at a time until I found the solution.
The downside is that you will need to review your typing to see if anything has a dotted red underline indicating that there is a potential misspelling. This is also annoying, so I’ll have to see which problem is more annoying over time.
I wish Apple just let us remove/ block specific words and phrases from the device’s dictionary. I never need to use “2.0” and would gladly have it treated as a misspelling on the rare occasion that I need to type it.