Clarification on the release notes of postgresql 12 regarding pg_upgrade

There are a few instances where the release notes seem to indicate that the
administrator should use pg_dump to upgrade a database so that improvements
on btree can be available.

Here are they:

1.

>In new btree indexes, the maximum index entry length is reduced by eight
bytes, to improve handling of duplicate entries (Peter Geoghegan)
-

> This means that a REINDEX
<https://www.postgresql.org/docs/12/sql-reindex.html> operation on an index
pg_upgrade'd from a previous release could potentially fail.
-
- 2.
>Improve performance and space utilization of btree indexes with many
duplicates (Peter Geoghegan, Heikki Linnakangas)
>...
>Indexes pg_upgrade'd from previous releases will not have these benefits.

3.
>Allow multi-column btree indexes to be smaller (Peter Geoghegan, Heikki
Linnakangas)
>...
>Indexes pg_upgrade'd from previous releases will not have these benefits.

My questions are:

1. Is this a current limitation of pg_upgrade that will be dealt afterwards?

2. Are we going to see more of such cases were pg_upgrade leaves the
database incompatible with newer features.

3. What's the recommendation for administrators with databases that are too
large to be upgraded with pg_dump?