In RHUI/PAYG, the term "custom repositories" has a special meaning which doesn't match what the -no-rhsm argument in leapp does. There are RHUI repositories, and also there are "custom repositories".  Any leapp wording which says smth like "-no-rhsm = custom repositories" should be changed.

Examples:
 1)  --no-rhsm             Use only custom repositories and skip actions with Red
                        Hat Subscription Manager

It can be changed to: "skip actions with Red Hat Subscription Manager and use any other repositories available on the system"

2)

Title: There are no enabled target repositories
Summary: This can happen when a system is not correctly registered with the subscription manager or, when the leapp --no-rhsm option has been used, no custom repositories have been passed on the command line.
Remediation: [hint] Ensure the system is correctly registered with the subscription manager and that the current subscription is entitled to install the requested target version 8.4. If you used the --no-rhsm option (or the LEAPP_NO_RHSM=1 environment variable is set), ensure the custom repository file is provided with properly defined repositories and that the --enablerepo option for leapp is set if the repositories are defined in any repofiles under the /etc/yum.repos.d/ directory. For more information on custom repository files, see the documentation. Finally, verify that the "/etc/leapp/files/repomap.json" file is up-to-date.