When blocklist v3 is enabled by default everywhere (desktop in bug 1631018, mobile in bug 1639050), the v2 implementation can be removed.

There isn't a separate bug yet, but Thunderbird should be given enough time to do the migration as well. Given their low volume of add-ons, I expect that simply converting all blocks to stashes (i.e. spreading all addon+ID combinations as JSON via remote settings) would work, without a need to develop a backend that generates a bloom filter (if this makes the migration easier).