Hi Even,

supporting a +towgs84 term in "old style" geodesic strings contained into spatial_ref_sys is absolutely intentional.

here is a short rationale:
1. when spatialite is linked against PROJ.6 or PROJ.7 all geodesic definitions are
   always retrieved directly from the PROJ own sqlite DB
2. so the old geodesic strings in spatial_ref_sys are still there just to support
   the case when spatialite is linked against some obsolete version of PROJ,
   and in this case defining +towgs84 is absolutely required in order to
   preserve a fair historical coherence.

future perspectives: when old PROJ versions will be reasonably disappeared, 
spatial_ref_sys will become just a plain dictionary (SRID, auth_name, auth_srid 
and ref_sys_name) mainly intended to support Foreign Key constrains in 
geometry_columns, but will no longer contain any geodesic data at all.