OP here. I found this URL by intercepting traffic on my older jailbroken iOS device. It's possible that a more recent version of iOS doesn't use it. This url, among others, was returned in some configuration-fetching call to https://api.smoot.apple.com/bag (which requires auth)

Here's some other ones:

- "correction_dict_url": "https://cdn.smoot.apple.com/static/static_corrections_dict/2..."

- "crowdsourcing_blacklist_url": "https://cdn.smoot.apple.com/static/crowdsourcing_blacklist_u..."

- "crowdsourcing_whitelist_url": "https://cdn.smoot.apple.com/static/crowdsourcing_whitelist_u..."

- "spotlight_model_resources": "https://cdn.smoot.apple.com/static/spotlight_model_resources..."

- "spotlight_stopword.map": "https://cdn.smoot.apple.com/static/spotlight_suggestions_sto..."

- "spotlight_phrase_dictionary.map": "https://cdn.smoot.apple.com/static/spotlight_suggestions_phr...

- "silhouette_topic_mapping": "https://cdn.smoot.apple.com/static/silhouette_topic_mapping/..."

- "silhouette_whitelisted_topics": "https://cdn.smoot.apple.com/static/silhouette_whitelisted_to..."

- "silhouette_config": "https://cdn.smoot.apple.com/static/silhouette_config/5/silho..."

- "dictionary_resources_url": "https://cdn.smoot.apple.com/static/dictionary_resources_url?..."