You may have noticed that your classic administration links are now missing from the SharePoint Administration center. Good luck editing your search schema or creating an app catalog now!

I have no idea why this was removed and hope this is corrected soon. Seems like a mistake to me even if the goal is to eventually roll out updated versions. Fortunately, if you happen to have the old links you can still visit them. Surely you bookmarked them all?!
No need to excessively weep! Reduce your wailing to dry sobs because here are the links to the missing admin pages. Copy them and paste them after your sharepoint admin domain (see below for an example).
infopath
/_layouts/15/TenantForms.FormServer.aspx
user profiles
/_layouts/15/tenantprofileadmin/manageuserprofileserviceapplication.aspx
bcs
/_layouts/15/bdc/TA_BCSHome.aspx
term store
/_layouts/15/termstoremanager.aspx
records management
/_layouts/15/TA_OfficialFileAdmin.aspx
search
/_layouts/15/searchadmin/TA_SearchAdministration.aspx
secure store
/_layouts/15/sssvc/TA_ManageSSSvcApplication.aspx
apps
/_layouts/15/online/tenantadminapps.aspx
sharing
/_layouts/15/online/ExternalSharing.aspx
settings
/_layouts/15/online/TenantSettings.aspx
configure hybrid
/_layouts/15/online/SharePointHybridSettings.aspx
access control
/_layouts/15/online/TenantAccessPolicies.aspx
data migration
https://docs.microsoft.com/en-us/sharepointmigration/introducing-the-sharepoint-migration-tool
With the exception of that last one, type in your sharepoint admin address (tenantname-admin.sharepoint.com) and then throw the link above after it.
For instance, here’s my classic search administration link:
https://thechriskent-admin.sharepoint.com/_layouts/15/searchadmin/TA_SearchAdministration.aspx

Update:
You can use this link to get to the original “More features” navigation page to make getting these links a little easier:
https://admin.microsoft.com/sharepoint?page=classicFeatures
Additionally, if you add ?showclassicnav=true to the end of any of the above links you’ll get that classic side navigation.