Why PostGIS sets a max value for SRID IDs?

[G. Allegri]

Hello list,

I'm working on two projects where custom CRSs, with custom authorities and
IDs are provided.
Both projects use IDs with numbers beyond SRID_USR_MAX=998999 [1], which is
hardcoded in PostGIS for the spatial_ref_sys id field values.
I can use the auth_id, of course, but having to reassign an id < 998999 is
a bit problematic for two reasons:

- I have custom transformation pipelines defined inside the proj.db, where
the custom IDs are defined for CRSs.
- other softwares (QGIS, Geoserver) can use the custom IDs but they cannot
match the geometries SRIDs returned PostGIS

I wonder if there's a technical reason for the SRID_USR_MAX constant, and
if there's any change to remove it in the future.
And I wonder if others have faced the problems I'm having due to this, and
what are the solutions they came up with.

Thanks,
Giovanni

[1]
https://github.com/postgis/postgis/blob/5dc95f1bc3047b048128616d4543b603b8bbdca7/configure.ac#L1554
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