pg_dump is not backwards compatible in PG12

PG 12 bumped the pg_backup_archiver version to 1.14, however this version
is not supported in past versions of pg_backup_archiver, resulting in
restores to earlier versions not working.

- Supported pg_backup_archiver versions in 12:
https://github.com/postgres/postgres/blob/REL_12_0/src/bin/pg_dump/pg_backup_archiver.h#L77-L103
- Supported versions in 11:
https://github.com/postgres/postgres/blob/REL_11_STABLE/src/bin/pg_dump/pg_backup_archiver.h#L77-L102

The following steps will reproduce the error:
1. pg_dump on a 12.1 database
2. pg_restore to a postgres db on a version < 12 (in my case 9.6). The
error: `pg_restore: [archiver] unsupported version (1.14) in file header`

Questions/observations:
1. Is it expected that pg_dump will produce backwards incompatible dumps?
If so, is this documented somewhere?
2. The release notes don't mention this at all as far as I could see.
Should they?
3. Should this be a bug? This is the first backwards incompatibility in
pg_dump (that I can see) since 9.6

Thanks,
Vera Johne