What is the 'tlv_payload' in lnd route?
The tlv_payload is a modification to the way onion packets are created between the 1.0 version and the 1.1 version (current working version) of the BOLT specs. The (optional) feature is named var_onion_optin in BOLT#9.

In V1.0 of the spec, there was a fixed structure for sending onion payloads to hops along a route, which contained a short_channel_id, a cltv_expiry and an amount_to_forward. In the latest version, implementations can add custom data to be forwarded in addition to the previously required ones. These contain a type and length encoded as a compact size integer, and the value of length bytes is serialized afterwards. Hence Type-Length-Value, or TLV.

Nodes which support the newer format advertise the var_onion_optin feature when they broadcast a node_announcement message, by setting bit #9 in the features field of this message.

In the lnd codebase, the feature folder contains the logic for feature bit vectors. It appears from manager.go that LND now treats the tlv_payload as enabled by default unless it is disabled as part of the config. Older versions of LND will obviously not support the feature and will use the legacy hop_data payload format.