I couldn't find any information on what's the minimum Salesforce license to grant a support person permissions to do Subscriber Support in the LMA.
Can someone please point me to such information?
I don't think there's an explicit document that states this for sure, but checking a few different type of profiles based on license types, it appears that the Log in to Subscriber Org permission can only be set on Salesforce licenses. I don't have access to the more esoteric licenses, so I can't confirm all of the license types, but it seems that you need to use a license that has at least core CRM functionality (Leads, Accounts, and Contacts). Chatter, Community, and Portal users don't have the necessary core features to use LMA.
After a dance with Salesforce support they confirmed that in Salesforce there is no single document stating what is needed for that but here it is:
Sales Cloud Standard UserLicense, Sys Admin not needed
Leads, View All is not required
Log in to Subscriber Organizationsystem permission (a permission set is enough for that)