I have the following scenario inside our sharepoint online and sharepoint on-premises 2013:-
1) We have an enterprise wiki site collection inside our sharepoint on-premises 2013. the enterprise wiki have 6 managed metadata columns which allow multiple values as follow:- 
2) Now when i go to the Wiki Pages library >> Add Indexes >> i were not able to add indexes for any of the managed metadata which allow multiple values, and seems sharepoint does not support indexing managed metadata column which allow multiple values. so currently we only have these indexes:-
3) I have enabled the metadata navigation for the wiki pages list and i added the managed metadata fields to them.
now we want to migrate the above enterprise wiki site collection to sharepoint online, so what are the appraochies we can have to allow indexing the managed metadata columns which allow multiple values? so if the wiki page library exceeds the SP online threshold limit (which is still 5,000), then we can break our main list view into smaller ones using the managed metadata fields? also to allow the Managed Navigation to keep working as well..