Goldenrod has reached its end of life and is no longer supported.

It isn't clear from the issues and whether some solution for the specific migration from Fameflower to Goldenrod is needed, or whether this is about migration from one release to the next release in general.

I was thinking more on the lines of migration from one release to the next in general.

That assumption hasn't changed.
If you haven't migrated your live production system from Fameflower to Goldenrod the index preparing pre-release might still be needed.
It isn't clear from the issues and whether some solution for the specific migration from Fameflower to Goldenrod is needed, or whether this is about migration from one release to the next release in general.

: My understanding based on your description in https://folio-org.atlassian.net/browse/MODINVSTOR-514 is that index rebuilding took a while between FF-GR due to move of functions from public to individual module schema and the expectation is that this might not repeat in future releases. Has that assumption changed?

: Please give more details how you would like this upgrade task been split up.
We may provide pre-release versions for mod-inventory-storage that only build new indexes.
For example

• 19.2.5 builds all new instance indexes needed for 19.3.*.

• 19.2.6 builds all new holdings indexes needed for 19.3.*.

• 19.2.7 builds all new item indexes needed for 19.3.*.

Installing these versions doesn't change any functionality compared to 19.2.4. When the underlying data changes both the old and the new indexes will be updated. Only the old indexes are used for queries.

When upgrading to 19.3.* no index rebuild is needed, the new indexes are ready to use, they will simply be enabled and the old indexes deleted.
You may install 19.2.5 on Wednesday night, 19.2.6 on Thursday night, 19.2.7 on Friday night, and migrate to 19.3.* on the weekend.
The total time is the same, but it is split and installing 19.2.5, 19.2.6 and 19.2.7 doesn't require any downtime.