[jcranmer]

I believe all scripts that are in extant use today are complete; if not, the missing extant scripts are down to the scripts with very few literate people (thousands or fewer).

Many of the additions today, barring emoji, are covering historical usage. This includes things like Medieval scribal annotations, a different set of numbers for the Ottomans, and the Mayan script. It will still be over a decade for the historical work to be complete, since there is often a lot of actual research that needs to be done to understand how an ancient writing system works, which has to come before you can even put together a coherent proposal for a new script.

[NelsonMinar]

I think that's a good framework to think about Unicode being "done". But even extant scripts aren't done; consider the Bopomofo additions here in Unicode 13. And it's not clear what "done" even means for Chinese characters.