Adding upon triatic's answer, Microsoft did intentionally remove the FTPS Credentials button as told on https://learn.microsoft.com/en-us/answers/questions/1342921/have-azure-removed-ftp-credentials-tab-from-azure

However, its still possible to get the credentials via either Azure CLI or Azure Powershell, as per https://learn.microsoft.com/en-us/azure/app-service/deploy-ftp?tabs=cli#get-ftps-endpoint

The FTPS and SSH credentials seem to be identical

Azure CLI

az webapp deployment list-publishing-profiles --name <app-name> --resource-group <group-name> --query "[?ends_with(profileName, 'FTP')].{profileName: profileName, publishUrl: publishUrl}"

Powershell

$xml = [xml](Get-AzWebAppPublishingProfile -Name <app-name> -ResourceGroupName <group-name> -OutputFile null)
$xml.SelectNodes("//publishProfile[@publishMethod=`"FTP`"]/@publishUrl").value 

Also, it is not noted on that article, but when using Azure CLI, you can add -s (--slot) to the command to specify a slot

az webapp deployment list-publishing-profiles --name <app-name> --resource-group <group-name> --slot <slot-name> --query "[?ends_with(profileName, 'FTP')].{profileName: profileName, publishUrl: publishUrl}"