Is ./*/ portable?

Short answer: YES

Long Answer:

The POSIX standard (from opengroup) states that / will only match slashes in the expanded file name. Since Unix/Linux does not allow / in the file name, I believe that this is a safe assumption on Unix/Linux systems.

From the bolded text below, it seems that even for systems that will allow / in the file name, the POSIX standard require that / will not be matched to such file.

On Windows, looks like / is not allowed in the file name, but I'm not an expert on Windows.

From Shell Programming Language § Patterns Used for Filename Expansion:

The slash character in a pathname shall be explicitly matched by using one or more slashes in the pattern; it shall neither be matched by the asterisk or question-mark special characters nor by a bracket expression. Slashes in the pattern shall be identified before bracket expressions; thus, a slash cannot be included in a pattern bracket expression used for filename expansion. ...

Additional Note - clarifying pathname:

The pathname is defined in 4.13, with explicit reference to pathname with trailing slash in General Concepts § Pathname Resolution.

A pathname that contains at least one non-<slash> character and that ends with one or more trailing <slash> characters shall not be resolved successfully unless the last pathname component before the trailing <slash> characters names an existing directory or a directory entry that is to be created for a directory immediately after the pathname is resolved. Interfaces using pathname resolution may specify additional constraints when a pathname that does not name an existing directory contains at least one non-<slash> character and contains one or more trailing <slash> characters.