Comment # 1 on bug 1180415 from Michael Andres

A list of packages dropped from the distro is provided by the openSUSE-release
package. Zypper cleans those packages provided as 'weakremover(PACKAGE)' unless
you disable the cleanup globally in /etc/zypp/zypp.conf by setting
'solver.upgradeRemoveDroppedPackages = false'.

To keep individual dropped packages you need to lock them like you did.

===
In the current TW 20210102 `zypper se -s ShellCheck` shows only a x86_64
package being available. Moreover, the i586 -release packages list ShellCheck
as being dropped:

 $ zypper se -s --provides 'weakremover(ShellCheck)'

 S | Name             | Type    | Version        | Arch | Repository
 --+------------------+---------+----------------+------+-----------
   | MicroOS-release  | package | 20210102-820.1 | i586 | tw-oss
 i | openSUSE-release | package | 20210102-820.1 | i586 | tw-oss

To me it looks like ShellCheck was intentionally dropped on ix86.