Where are you subscribing to your plan to test this? Giving the user a subscription from backend can cause problems with CBSubs Fields Integration as the user store itself may override whatever the integration sets. On frontend it won't change the fields value until their subscription is active.

The field itself and the create limit behavior is fully functional. Your issue is entirely with CBSubs Fields not setting the fields value so need to find out what's interrupting that. Easiest way to check for external conflicts is unpublish other plugins like CB Auto Actions then check if the field is updating when subscription is active.

Additionally you can opt to change the fields value using CB Auto Actions instead of CBSubs Fields using the below.

Plan Active
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
Condition 1
Field: Custom > Value
Custom Value: [var3]
Operator: Equal To
Value: PLAN_ID_HERE
Condition 2
Field: Custom > Value
Custom Value: [var2]
Operator: Equal To
Value: A

Plan Expired
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
Condition 1
Field: Custom > Value
Custom Value: [var3]
Operator: Equal To
Value: PLAN_ID_HERE
Condition 2
Field: Custom > Value
Custom Value: [var2]
Operator: Not Equal To
Value: A

Be sure to replace PLAN_ID_HERE with the actual id of your plan as found in CBSubs > Plans. Now you'd just use a Field action to set the fields value.

0 means unlimited. There isn't a way to turn off group creation without using the create access setting. Try having CB Auto Actions update the field value using the above instructions and see if that'll work for you. I'm not sure what's preventing CBSubs Fields from working in your case, but there has been no changes to that integration to suggest it shouldn't work.

If you don't want a basic membership to be able to create groups you should assign a usergroup to your other memberships and adjust the group create access permission accordingly.

If your other memberships are assigning group create limit fine you don't need to use CB Auto Actions. I suspect your other plans are working just fine and your issue was the misunderstanding of create limit of 0.

To test that your other plans are working fine subscribe then in backend edit the user and check their group limit field has the appropriate value set.