Custom migrate source plugin, extends FieldableEntity class, has migrate status of [error] No migrations found. Why?
I resolved my problem when I identified a mistake in my @MigrateSource annotation on my plugin. I had included my own custom migration module in the source_module entry of the plugin annotation. Here is what I did wrong:
/**
 * The 'mymodule_migrate_entityform' source plugin.
 *
 * @MigrateSource(
 *   id = &quot;mymodule_migrate_entityform&quot;,
 *   source_module = &quot;mymodule_migrate&quot;
 * )
 */

Well I looked at the api docs on source_module

Identifies the system providing the data the source plugin will read.


The source plugin itself determines how the value is used. For example, Migrate Drupal's source plugins expect source_module to be the name of a module that must be installed and enabled in the source database.

It didn't work because source_module was set to my D9 custom migration module, which is obviously not on the D7 source, so the migration just silently gets removed from the list of migrations.
So I simply deleted the annotation line. My working plugin annotation looks like this now, and the plugin extends FieldableEntity and works as I had hoped it would.
/**
 * The 'mymodule_migrate_entityform' source plugin.
 *
 * @MigrateSource(
 *   id = &quot;mymodule_migrate_entityform&quot;
 * )
 */

I hope my plugin here will prove useful to someone migrating from D7 entityform to D8/9 core contact module (with contact_storage contrib module). It works.