By default, LND cooperates with all gossip_timestamp_filter requests. Prior to v0.18.3, LND’s logic to respond to these requests looks like this:
LND loads all requested messages into memory at the same time, and then sends them one by one to the peer, pausing after each send until the peer acknowledges receiving the message. The peer can specify any filter, including one that requests all historical gossip messages to be sent to them, and LND will happily comply with the request. As a result, LND can load potentially hundreds of thousands of messages into memory for each request. And since LND has no limit on the number of concurrent requests it will handle, memory usage can get out of hand quickly.