Reposting this here from the Discord server as requested:
When creating a collation, in https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/pg_locale_icu.c#L461 it is opening the collator with the tailoring rules supplied. However, it has hardcoded the strength level UCOL_DEFAULT_STRENGTH. This has the effect of ignoring the "deterministic=false" you may have specified in your CREATE COLLATION call. If, instead of UCOL_DEFAULT_STRENGTH, the code understood the deterministic parameter and passed either UCOL_PRIMARY for "deterministic=true", and UCOL_SECONDARY for "deterministic=false", this would preserve the attempt to obtain case-insensitivity in the locale while simultaneously allowing tailoring as expected.
I have made the modification to the pg_locale_icu.c and tested it locally (simply hardcoding UCOL_SECONDARY – not checking the deterministic parameter) and it behaves as expected, though I freely admit my knowledge of ICU intersecting with Postgres is rather limited.
