LDAP feature was gone after upgrading my On-Prem deployment
First, be sure that your license is applied correctly to your Workspace. After release 4.X.X, LDAP became an exclusive feature for the "Enterprise" plan.
Go under Administration > Info and check for the orange "Enterprise" tag.
If you don't have the tag, look for one of these probable solutions below.
If you have internet access, try to log in to our Cloud and Sync.
Go under Administration > Connectivity Services.
Log into Rocket.Chat Cloud with the same account used to create your Workspace.
Hit Sync button
Check the Info page again.
You don't have internet access (Air-Gapped Operation).
If you lost the feature after an upgrade, you must contact our Support Team and ask for an updated License that reflects the version you are using and apply it offline.
Go under Administration > Info and click on "Apply Offline License"
Copy and paste the license that you have been provided