The current 4.0.x branch is the version for both Drupal 8 and Drupal 9.

Please not that the unistallation of the Flag Lists module removes
all of your flags, NOT ONLY the flag lists related ones. This seems to
be a problem with Drupal itself!

There are menu items to administer and use the Flag Lists module.
There is view support for the entities of interest. ("flagging collection"
and "flag list item"). There is also a block view for your flag
list items.

Upgrade from D7 is working if the documentation on 
the Flag Lists documentation pages are followed.

ENTITIES

The definition of the different Entities used are as follow:

"flag_for_list" is a config entity which is handle from flag module
indirectly. It contains  which flags that are available as base flags,
i.e. how the flag links are defined on each entity. During upgrade
you will get warnings that the Base Flag doesn't exist. Don't use these
without fixing them first!

"flagging_collection" is the named list of one or more flags. It has
revisions, bundels and fields. The later defines how the collections
are handled and displayed.  The bundels are defined in the
"flagging_collection_type" entity. Each flagging collection is mapped
to a specific flag. This flag is a copy of the choosen template flag.

"flagging_collection_type" is the bundles for the "flagging_collections"

"flag_list_item" is the mapping between entities (bundles, id),
flagging_collection and flagging (flag id)

