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 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?