Acknowledge what they lost
Say what the feature did for them in their own terms before you explain anything.
Explain the reason briefly
Give the honest reason, such as low use or maintenance cost, without blaming the customer.
Offer a workaround
Show another way to reach the same goal, with steps, even if it is less convenient.
Pass the feedback on
Tag the ticket and send the count to product. Do not promise the feature will return.
Check if it affects their plan
If the feature was why they bought, ask what they need and respond under your normal cancellation policy.