For example ad-block is currently on format v4. That's where this 4 comes from in:
~/Library/Application\ Support/BraveSoftware/Brave-Browser-Beta/cffkpbalmllkdoenhmdmpbkajipdjfam/1.0.133/4/ABPFilterParserData.dat

The Brave Browser is smart enough to know to update the extension to v5 if it has client code from brave/ad-block that specifies the current version as v5 format.

The problem is, we need to go to version 5 format soon. This wouldn't be a problem for it landing in master only. But to do this we need to be able to keep old installs still working.

we'd want the ability to update extensions differently depending on the Chromium version in use.

So mainly we'd like to have the ability to have Chromium 73 and below use a differently packaged format v4 extension. And Chromium 74 and above use a differently packaged format v5 extension. Both would keep the same extension ID.

This would need changes for both brave/brave-core-crx-packager and brave/go-update. Mainly we'd just coordinate ad-block format updates on major chromium update releases so it works out nicely.