Should we flag safeguards that have been exceeded?

[eskibars]

For example, if a user has a large shard count (elastic/elasticsearch#17396), a large number of fields in their mapping (elastic/elasticsearch#17357), a large number of nested fields (elastic/elasticsearch#14983), or a very high depth in their mappings (elastic/elasticsearch#17400), they may switch over to 5.0 and have unexpected results. We could detect this in the migration assistant prior to the upgrade and provide some help.

[clintongormley]

Fixed in 2.0.0-beta1