The backzone zones cover only a very small part of the problems we know about.

Using the backzone prevents using the official tzdata distribution, as one has to compile with special procedures so that backzone data are compiled in.
This would prevent us from using the official tzdata packages provided automatically for Redhat Enterprise Linux.

On 03/06/2020 11:01, Alois Treindl wrote:

It is known that pre-1970 time zone history is represented in TZ database only for the cities which define the zone names.
Other areas of countries are often not correctly represented in TZ database before 1970.

In Europe, there are several rules pushed back to the backzone file which are accurate for all of the times back to the introduction of 'standard time' but if backzone is omitted from the TZ data then those areas fall back to the general rules from the current set prior to 1970. In particular the data during the Second World War for The islands around the UK is missed. The amount of missing data is only small, but the fact that it is missing is not always obvious?