Hi,

as reported in bug #1146072 libcapi20-3 FTBFS (#1143331) because
./configure hard-requires <linux/capi.h>, which linux-libc-dev no longer
ships since the kernel's ISDN/CAPI subsystem was removed upstream, so
the package is permanently unbuildable and is already marked for
autoremoval from testing on 2026-08-30.

The only reverse dependency I'm aware of is wine (libwine), whose
Build-Depends on libcapi20-dev is optional (WINE_NOTICE_WITH in
configure.ac): without it, configure just prints "libcapi20 development
files not found, ISDN won't be supported" and continues, skipping only
the dead capi2032.dll ISDN wrapper; dropping libcapi20-dev from wine's
debian/control is a one-line fix independent of this removal.

Please remove the Build-Depends in the next revision of wine to
solve this issue.

Kind regards
    Andreas.