Markus Schaber:

It is a common situation that some project allows for multiple alternative
licenses, some of them are "free" and expressible via SPDX, while others of
them are proprietary.

...

As the free licenses are always a legitimate choice for the users and
redistributors of those packages, I propose that to this express via a special
(reserved) identifier "CustomAlternateLicensing".

In a SPDX license expression you could use "OTHER", e.g.: (AGPL-3.0-only OR OTHER). That's less specific, but it does provide *some* information, it and doesn't require that a SPDX document exist.

As Sam Ellis noted, if you use a SPDX document you could name custom licenses and that would completely resolve the issue. However, that will only help you if you use SPDX documents. (As I've noted elsewhere, I think most SPDX users only use SPDX license expressions, and do not create or use full SPDX documents.)

--- David A. Wheeler