For many databases indexes have both benefits and drawbacks. An index can benefit a search, whether that my a Cypher 'match' statement or a SQL 'select' statement. However indexes can also slow an insert/update whether a Cypher 'create (n:Person.... or a SQL 'INSERT INTO Person......
But if the index is unused, i.e. its not there for the benefit of the search statements, then yeah removing would at least benefit your insert/updates.
Regarding
This post Neo4j 3.5.0 non-existing index exception after dropping indexes makes me a bit nervous..
there have been many patches to the 3.5.x branch, with the current being 3.5.7. If you are going to move to 3.5.x you would be better served taking the most recent